Nakajima B5N2 “Kate” Type 97-3 Carrier Attack Aircraft

Torpedo and Level Bomber

“Japanese carriers embarked two kinds of bombers. First, they had dive bombers, which they designated kanjo bakugekki-ki (carrier bombers.) Diving steeply, they dropped their bombs at close range for accuracy. Of course, this put them deep into anti-aircraft fire. Dive bombers usually could only carry a single medium-size bomb.”

Header photo source: U.S. Navy photo 80-G-427153 from the U.S. Navy Naval History and Heritage Command. Captured Kate 1943.

Raymond R. Panko
Ray@Panko.com
August 2017


Key Points >> Introduction >> The Aircraft >> Crew >> Pearl Harbor >> Development >>
Historiography >> Characteristics >> Japanese Naval Aircraft Designations >> References


Key Points

  • The Japanese called this type of aircraft a kanjo kōgeki-ki.
  • In English, this meant “carrier attack aircraft.”
  • Informally, a kankō.
  • Most navies would call it a torpedo bomber.
  • This kankō had three main designations.
  • Manufacturing project designation: B5N2.
  • Allied code name: “Kate.”
  • Official operational designation: Type 97-3 Carrier Attack Aircraft.
  • Best torpedo bomber in the world at the beginning of the Pacific War.
  • In the first wave at Pearl Harbor:
  • 40 Kates attacked with aerial torpedoes, sinking four battleships (California, Oklahoma, West Virginia, Nevada).
  • 49 B5N2s attacked with high-level bombs, destroying the Arizona with a crucial hit.
  • In the second wave, 54 attacked airfields, each dropping one 534-lb. bomb and either a second 534-lb. bomb or six 133-lb.
  • Crewed by a pilot, a teisatsu (observer), and a radio operator/rear gunner.
  • Teisatsu was an observer, navigator, bombardier.
  • Often (but not always) the senior crew member; if so, the aircraft commander.
  • Good bomb load and range.
  • Slow cruise speed (161 mph), no armor or self-sealing fuel tanks, only a single rear gun for defense.
  • After losses became unsustainable, moved to rear-area sea lane control duties in 1944.

Introduction

At Pearl Harbor, Japan’s most devastating aircraft was the Nakajima B5N2, also known as the “Kate” and the Type 97-3 Carrier Attack Aircraft.

  • In the opening minutes of the attack, 40 Kates savaged Battleship Row with torpedoes. When they finished, Oklahoma and West Virginia had sunk, and California and Nevada were sinking.
  • Immediately afterward, 49 more Type 97-3s appeared. They flew along the backbone of battleship row at almost 10,000 feet. Each dropped a massive 800-kg (almost 1,800-lb.) class bomb designed to penetrate a battleship’s armored deck and ignite a magazine. One of these bombs destroyed the Arizona in a blast heard for miles.
  • In the second wave, 54 more B5N2s arrived. This swarm ignored ships and attacked airfields. Each dropped two or more bombs, bringing heavy devastation and loss of life.

Pearl Harbor was just the beginning. Kates quickly proved that they could sink maneuvering ships at sea as well as ships sitting in harbor. In the first year of the war alone, they sank or helped sink three U.S. carriers: Lexington, Yorktown, and Hornet [Aireview staff]. A postwar summary[46], concluded that, “The achievements of Kate were compatible with those of the Zero.”

Although the Type 97-3’s level bombing attacks were ineffective against ships at sea, Type 97-3s used level bombing to support Japanese troops in Japan’s lightning advance through the South Pacific.

Carrier Bombers and Attack Aircraft
Japanese carriers embarked two kinds of bombers. First, they had dive bombers, which they designated kanjo bakugekki-ki (carrier bombers.) Diving steeply, they dropped their bombs at close range for accuracy. Of course, this put them deep into anti-aircraft fire. Dive bombers usually could only carry a single medium-size bomb.

The other type of bomber on Japanese carriers was the kanjo kōgeki-ki. (Kan meant ship, jo meant embarked, kogeki meant attack, and ki meant machine — in this case an aircraft.) Therefore, it was a “carrier attack aircraft.” Informally, Japanese crews shortened this to kankō. Other navies called such planes torpedo bombers. However, neither torpedo (gyorai) nor bomb (bakudan) appeared in the Japanese designation. Like torpedo bombers in other navies, kankōs attacked with either torpedoes or bombs. “Attack aircraft” reflects this versatility. The kankō at Pearl Harbor had three times the bomb capacity of the dive bomber. It always dropped its bombs in level flight, increasing safety but limiting accuracy.

What I Call the Kankō that Attacked Pearl Harbor in this Study Report
Japan’s kankō when the war began was generically designated the B5N and the Type 97 Carrier Attack Aircraft. Its first version was called the B5N1 and the Type 97-1 Carrier Attack Aircraft. The second, which was used at Pearl Harbor, was called the B5N2 and the Type 97-3 Carrier Attack Aircraft. Like the Japanese, I use the term Type 97-3 rather than giving the whole name when context makes “Type 97-3” unambiguous. In the Allied code name designation system, both versions were called the Kate.

Referring to Japanese aircraft is problematic because the Japanese designation systems in World War II were complex, changed frequently, and used inconsistently [Francillon 1995 46-59, Mikesh 170-181]. If you are familiar with Japanese aircraft designations, you probably want to skip the Appendix on Japanese Naval Aircraft Designations, although you might want to read why I call the kankō that attacked Pearl Harbor the Type 97-3 Carrier Attack Aircraft instead of the Type 97 Model 12.

The Aircraft

The Type 97 kankōs were single-engine, low-wing monoplanes with well-cowled engines and flush-riveted skin to minimize aerodynamic drag [Francillon 1969 61]. Both had a crew of three seated one behind the other. The cowling was an NACA (U.S. National Advisory Committee for Aeronautics, NASA’s predecessor) design that also minimized drag [Aireview staff 45, Hawkins 4].

Fuselage
The Kate’s fuselage had an oval cross-section and used semi-monocoque construction. In a pure monocoque construction, the outer skin supports the aircraft structure completely, like an egg shell. In semi-monocoque construction, the skin is strengthened by adding internal ribs. Even with these ribs, semi-monocoque construction is lighter than a tubular structure.

Figure 1: Semi-Monocoque Fuselage Construction Figure 1: Semi-Monocoque Fuselage Construction

Source: Ray@Panko.com, taken at Pacific Aviation Museum Pearl Harbor.

Like torpedo bombers in other navies, the Type 97-3 had a crew of three. Figure 6 shows that the members of the Type-97 crew sat under a single glazed canopy. Each could open the canopy around his station independently [King 133]. Each had a bucket seat with a seat belt [King 133]. There was no need for shoulder harnesses because the Kate did not engage in violent maneuvering like the Zero [King 133]. The crew communicated via unpowered speaking “Gosport” tubes [Hawkins 6, King 152, Mori 1323].

Crew Seating Figure 2: Crew Seating

Source: San Diego Air and Space Museum Archive. Catalog Number 01_00086081.

Wings
The B5N wings had a flush-riveted stressed metal skin with all-metal flaps and fabric-covered ailerons [Francillon 1969 61 1995 415]. The slotted flaps extended to the wing fold, the ailerons from the fold outward [Hawkins 9]. These big wings gave the B5N the lift capacity it needed to carry its big ordnance loads. In addition, their size gave ample room for the Kate’s wide-stance hydraulic inward-folding landing gear attached to the main spar. This was the first use of hydraulically folding landing gear on a Japanese single-engine aircraft [Aireview staff 45]. Size also gave room for fuel tanks between the wing’s two strong spars [Hawkins 9].

The B5N2’s Long Wingspan The B5N2’s Long Wingspan

Source: U.S. Navy Photograph of a B5N2 descending after having been fatally damaged by a PB4Y patrol bomber.

Underside of Type 97-3 Wings Figure 3: Underside of Type 97-3 Wings

Source: Ray@Panko.com, Taken at the Valor in the Pacific Memorial. The exhibit is a fiberglass model.

For carrier deck and hangar storage, the Kate’s wings folded upward. Jury struts were attached to the fuselage to support the weight of the folded wings during storage [Hawkins 9]. To minimize folded height and width on hangar decks, the right wing folded partially under the left [Hawkins 5]. To develop this low-wing design, Nakajima used knowledge gained earlier by examining designs from Northrup, Douglas, and Clerk [Aireview staff 45, Hawkins 10], but the wing design was not a mere copy of foreign aircraft wings. The tail had fabric-covered control surfaces [Hawkins 10].

Overlapping Wings for Storage Figure 4: Overlapping Wings for Storage

Source: National Archives Photograph in Francillon [1969].

Ordnance Carriage
Kates carried their ordnance externally, under the fuselage. They had different racks for different ordnance loads [Hawkins 6], which were typically a single torpedo, a single 800-kg class bomb, two or three 250-kg class bombs, or six 60-kg class bombs. In China, two 250-kg class bombs or six 60-kg class bombs dominated ordnance selection [Aireview staff 46]. For some bombs, there were front and back fuzes, which could be selected by the bombardier before the drop [Panko Fuzes]. One fuze detonated the bomb on contact, while the other delayed the explosion about 0.2 second. The former was best for outdoor targets, the latter for penetrating buildings before exploding. The variety of weapons racks could cause re-arming issues in the stress of combat, such as at Midway.

Kate with Bomb Load. Probably with three 250-kg (550-lb.) bombs Figure 5: Kate with Bomb Load. Probably with three 250-kg (550-lb.) bombs

Source: Imperial Japanese Navy via U.S. Navy

Engine and Performance
The B5N2 used a Nakajima 14-cylinder, two-row, air-cooled radial engine, the Sakae 11 [Francillon 1995 414]. The Imperial Japanese Navy used two engine designation systems. Sakae 11 was the operational designation [Francillon, 1995 515]. The manufacturing project designation was the NK1B, where N meant Nakajima, K meant air cooled, 1 meant that it was the first air-cooled engine in the current numbering sequence, and B meant that it was the second version of the engine [Francillon, 1995 515]. As in aircraft designation, Japanese engine designations were complex and changed over time [Francillon, 1995 515].

The Sakae 11 generated 1,000 hp at takeoff and 970 hp at 9,845 ft (3,000 m) [Francillon 1995 415]. It gave a top speed of 235 mph at 11,810 ft and a cruising speed of 161 mph at 9,845 ft [Francillon 1995 415]. Carrying a heavy bomb load reduced these numbers, but the Kate was still faster than America’s torpedo bomber at the start of war in the Pacific, the Douglas TBD Devastator, and the main British torpedo bomber in 1941, the Fairey Swordfish biplane [Francillon 1969 16]. America soon introduced the excellent TBF/TBM Avenger torpedo bomber, but America’s defective aerial torpedo meant that USN torpedo attacks still brought little profit until the torpedo was greatly modified in late 1943 [Panko Torpedo].

Defensive Limitations
Given the Kate’s limited engine power, Nakajima could not add armor to the Kate because this would seriously degrade speed. The need to carry enough gasoline to meet range requirements, in turn, precluded self-sealing fuel tanks. Their thick rubber bladders reduced fuel volume too much. (When the Kate’s successor, the Tenzan, used self-sealing fuel tanks in a prototype, fuel capacity was reduced by 30 percent [Francillon 1995 431].) Although the Kate was rightly called the best torpedo bomber in the world at the start of the war in the Pacific, it was a slugger with a glass jaw.

If fighter support was not present, a flight of Kates was easy prey for enemy fighters. Coming in from the front, a fighter could attack with impunity because B5Ns had no forward-firing machine guns. One Type 97 pilot who faced a head-on fighter attack reported that Wildcat fighters attacking from the front destroyed his nine-ship hikōtai completely, its pilots being unable to do anything but try to ram the attackers [Mori 3549]. Even attacks from the rear only faced a single 7.7 mm (.303-caliber) Type 92 machine gun. Even several Type 97 flying in close formation posed limited risk to attacking fighters.

The Crew Roles

The Japanese Navy used many terms based on English. The flight crew of an aircraft was called a peah (pair) regardless of its size [King 140]. The Kate peah consisted of a pilot, teisatsu (observer), and a radio operator/gunner. Each had important roles during an attack.

Crew Seating Figure 6: Crew Seating

Source: San Diego Air and Space Museum Archive. Catalog Number 01_00086081.

The Pilot
The pilot’s seat back attached to a pole, allowing the him to raise his seat for takeoff and landing. This improved visibility the over the long nose of the big tail-dragger. When the seat was at its highest elevation, the pilot’s head was just below the top of the windshield [Hawkins 6].
Pilot's Seat
Figure 7: Pilot’s Seat

Source: Imperial Japanese Navy via Burin Do 1992.

Japanese fighter pilots dismissed B5N pilots as “cart drivers” because the big kankō was slow and had little maneuverability [Mori 22 4041]. However, flying the Kate required extreme concentration and split-second decision making. The pilot had 16 instruments to monitor constantly, had to fly in close formation, and had to fly very smoothly because variation in engine power caused fuel burn to spike in a heavily loaded airplane [Mori 691]. In addition, during a torpedo or bomb drop, it was essential to fly absolutely level, with no pitch, yaw, or roll. Any small variation would throw the bomb off target. When Juzo Mori attacked the California, he had to make a series of split-second decisions about not attacking the Helena, aborting his first run at the California, and swinging around to attack the California again, this time successfully [1809-1840]. He then had to weave an escape route through heavy anti-aircraft fire seeking revenge [1845]. He had to do all this flying at very low level in an aircraft of limited maneuverability.

Radio Operator / Machine Gunner
In the rear sat the radio operator/machine gunner, who keyed messages in Morse code [Mori 1614]. The radio in a Kate was fairly good. This radio was much better than the miserable set in Zeroes, so to return to the striking force, Zeroes and Kates rendezvoused, and the B5N2s shepherded the fighters back to the carriers. When Mitsuo Fuchida had his radio operator break radio silence and send the message Tora, Tora, Tora to his carriers to advise that surprise had been achieved, the transmission was heard in Japan, although only because of odd atmospheric conditions [Fuchida 1952, King 149].

The rear-seater had a rather cheap-looking folding chair that faced forward for radio operation [King 133]. During the Pearl Harbor attack, there was radio silence, so the radio operator/gunner focused entirely on gunnery [King 134]. To use the machine gun, the rear-seater folded his chair, clipped it to the side of the cockpit, unstowed the gun, and stood up to fire [King 134]. (On Val dive bombers, in contrast, the rear seat swiveled front to back [King 134]). He was normally tethered to the floor by his parachute cord, but crews did not use parachutes during the Pearl Harbor attack, so he had no attachment to the aircraft beyond his hands on the gun and his knees braced against the side of the fuselage [King 148].

The weapon itself was a Type 92 7.7 mm (.303 in) machine gun with six 97-round drum magazines—one on the gun and five spares [Mori 1702]. Based on the British Lewis machine gun used on World War I aircraft, it fired 600 rounds per minute, and its muzzle velocity of 2,500 ft per second gave it an effective range of 600 m [Francillon 1995 431]. Its arc of aim was about forty degrees left, right, and down and about 80 degrees upward [King 134]. As already noted, this gun was puny compared to those in American fighters, most of which had six .50 caliber machine guns with larger bullets, greater range, and a much higher volume of fire.

Type 92 Machine Gun (7.7 mm, .303 in)
Figure 8: Type 92 Machine Gun (7.7 mm, .303 in)

Source: Imperial Japanese Navy via Burin Do 1992.

Side View of Captured Type 97-3 Carrier Attack Bomber Showing an Unstowed Machine Gun Figure 9: Side View of Captured Type 97-3 Carrier Attack Bomber Showing an Unstowed Machine Gun

Source: National Archives Photograph in Francillon [1969].

Teisatsu
The middle-seater had the most complex job. He was the teisatsu, which is usually translated as observer [King 130]. On reconnaissance and maritime patrol missions, observation was, in fact, his main task. To improve his view, he could raise or lower his seat up to about 14 inches, and swivel his seat a bit over 35 degrees right or left [King 133]. He also had two small windows in the fuselage sides to illuminate his charts and manuals.

Navigation
However, observation was only one aspect of the teisatsu’s job. Most importantly, he was the aircraft’s navigator, which was a critical task for long-distance missions over water. Chief Petty Officer 1st Class Haruo Yoshino, the teisatsu on a Kaga Kate in the Pearl Harbor attack, noted that he had been trained in many navigation tools, including the use of charts, maps, slide rules, sextants, and navigation by the stars [King 130]. On missions, he brought a large bag called a yōgubukurō, which stored his navigation gear this plus binoculars, flare gun, and possibly an aerial camera [King 130]. Access to a downward photo-reconnaissance camera was also available from this position.

Dropping Bombs
On level bombing missions, the teisatsu was the bombardier. To sight the target, he had two trap doors on the left side of the floor. He unstowed his Type 90 telescopic bomb sight and lowered it into one of the doors [King 132]. As the aircraft neared the target, the teisatsu directed the pilot to go slightly to the left or right. When the bombsight was lined up with the target, the teisatsu released the bombs [King 132]. The bombardier preceded the drop by a steady vocalization, which he changed to a shout at the moment of drop [King 132]. Because the doors were on the left side of the cockpit, the bombs (or torpedo) were offset to the right side of the fuselage. A five-plane formation arranged in a V was called a “buntai,” and the lead Kate’s bombardier dropped first, with the other four aircraft toggling simultaneously.

Underside of Type 97-3 Wings Figure 10: Underside of Type 97-3 Wings

Source: Ray@Panko.com, Taken at the Valor in the Pacific Memorial. The exhibit is a fiberglass model.

Torpedo Attacks
In torpedo attacks, the teisatsu’s job was more passive. As the aircraft neared potential targets, the teisatsu, who had time to look around, selected a target and directed the pilot to it. Teisatsu Haruo Yoshino originally selected the West Virginia, but the ship was blanketed by water spray from other torpedoes. He directed the pilot to switch to the Oklahoma [King 150]. Either the pilot or the teisatsu could release the torpedo. At Pearl Harbor, Yoshino performed the release, freeing the pilot to concentrate on low-level flying [King 151]. After the torpedo release, the teisatsu told the pilot which way to turn to escape.

Although the teisatsu’s role in torpedo attacks was important, the pilot was in control of the torpedo attack [Mori]. Only he could aim the aircraft at its target, using a sight on the top of the instrument panel [King 133]. Only he had the perspective to know when to drop the torpedo, especially at sea, where almost every torpedo release involved complex deflection shooting at a precise speed and altitude against a moving target. He had to make these calculations mentally while dodging flak and flying the airplane at low altitude [King 133].

Landings
Carrier landings are always difficult, especially for heavy bombers that lack agility. To ease the pilot’s mental load, the teisatsu constantly called out instrument data during the landing [King 138].

Who Was in Charge?
The pilot and teisatsu were usually noncommissioned officers, while the radio operator/gunner was an enlisted sailor. When the teisatsu was the senior member of the crew, he was the aircraft commander or kichō [King 130]. In the Pearl Harbor Raid, Commander Mitsuo Fuchida led the entire attack from the teisatsu position of a Kate [Fuchida 2011]. On the way toward Oahu, he navigated for the entire first wave [Fuchida 2011].

Kates in the Pearl Harbor Attack

The objective of the Pearl Harbor attack was to keep American battleships from interfering with Japan’s main goal—its thrust south to the oil fields of Borneo and the Dutch East Indies [Fukudome]. Both sides envisioned an eventual climactic slug-out between battleship that would decide the course of the war. With fewer battleships, Admiral Husband E. Kimmel would be outnumbered and would not sortie his battleships at the start of war per the Rainbow 5 War Plan. Figure 11 shows where the battleships were moored on the west side of Ford Island. One battleship not on Battleship Row was Pennsylvania, which was in dry dock being overhauled. Pearl Harbor’s ninth battleship, Colorado, was in Bremerton Washington being overhauled [Yarnell].

Battleship Row, Carrier Berths, and Seaplane Base with PBY Patrol Bombers Figure 11: Battleship Row, Carrier Berths, and Seaplane Base with PBY Patrol Bombers

Sources: Contemporary NASA Satellite Photo, Ray@Panko.com

The Imperial Japanese Navy also knew the importance of carriers. Figure 11 shows the normal mooring spots of the three carriers stationed at Pearl Harbor. Note that Enterprise normally moored directly in front of California. The Lexington and Saratoga, in turn, usually berthed on the West Side of the island [Panko Enterprise]. There was a Battleship Row, but there was no carrier row. The Japanese knew that no carriers had been reported to be in harbor, but they still had these sixteen Kates attack from the west.

Enterprise and Lexington were away on missions to deliver airplanes to Midway and Wake [NHHC], there being no escort carriers available yet. Enterprise had actually been scheduled to return to port the day before the attack, but it was delayed by heavy seas [Bureau of Ships 3]. Pearl Harbor’s third carrier, Saratoga, was entering the harbor at San Diego to pick up her airplanes after it had been refitted in Bellingham, Washington [NHHC].

The Torpedo Attack
Torpedoes were the only reliable way to destroy battleships. The mine effect of the warhead’s explosion could break open a battleship’s side, especially if it hit below the armor belt. Japan’s Type 97 Modification 2 (not Model 2) aerial torpedo (koku gyorai) was certainly the best aerial torpedo in the world at the timer. It weighed 1,840 lb and had a 610 lb warhead with an explosive charge of 450 lb [NTMJ Aerial Torpedoes]. This was far more powerful than any bomb dropped on Pearl Harbor. Best of all, it was reliable, having been relentlessly tested and developed since 1931. Finally, it was delivered by superbly trained aircrews from Akagi, Kaga, Sōryū and Hiryū. The green aircrews of the newly worked up Shōkaku and Zuikaku would be in less vital parts of the attack.

Due to their weight and speed, when the aerial torpedoes struck the water, they kept diving down. At shallow Pearl Harbor, this initial plunge would have driven the Type 91 Mod 2 into the mud. The Japanese worked furiously to modify the torpedo to pitch up immediately upon entering the water instead of waiting for it to right itself after water entry. (The torpedo was heavier on the bottom than the top, but this natural and automatic method for righting the torpedo took time that the plunging torpedo did not have.)

The solution was a gyroscope that controlled two ailerons at the front to the tail cone. This anti-roll mechanism ensured that the torpedo would be upright when it struck the water, allowing the horizontal rudders to be pitched up upon water entry without the danger of throwing the torpedo left, right, or even down. The big stabilizing fins at the very back of the torpedo were there to reduce wobble, not to pitch the torpedo up or down as it dropped through the air [Panko Torpedo].

Japanese Type 91 Modification 2 Torpedo with Anti-Roll Fins (Left) and Stabilizing Fins (Right)
Figure 12: Japanese Type 91 Modification 2 Torpedo with Anti-Roll Fins (Left) and Stabilizing Fins (Right) and

Source: Photograph Taken at Pacific Aviation Museum, Pearl Harbor.

Rear Horizontal Rudder Effect when Torpedo is Upright on Water Entry (Unrolled) Figure 13: Rear Horizontal Rudder Effect when Torpedo is Upright on Water Entry (Unrolled)

Source: Panko [Torpedoes].

Rear Horizontal Rudder Effect when Torpedo is Not Upright (Rolled) on Water Entry Figure 14: Rear Horizontal Rudder Effect when Torpedo is Not Upright (Rolled) on Water Entry

Source: Panko [Torpedoes].

Torpedo attacks were extremely risky. The Kates had to approach flying steady at wave-top level and low speed. If the ships were alerted and fully armed, their guns could savage torpedo attackers. Consequently, torpedo attack at Pearl Harbor was slated to occur at the very start of the assault so they could strike before American guns were crewed and ready, but a mix-up in signals led to dive bombers attacking the PBYs on the southern tip of Ford Island two minutes before the first torpedo bombers arrived at 7:57 [Aiken].

Despite this near-absence of warning, some of the guns on the battleships quickly got into action. In fact, several pilots and teisatsus marveled at the amount of fire they received [Aiken, Fuchida 1952, King 152-153]. Since April 1941, each battleship constantly manned two 5-inch dual-purpose guns and two .50-caliber machine guns [Gannon 545-546]. The 5-inch guns had a locked but quickly accessible, ready supply of 15 shells [Gannon 545-546, Wallin 106]. The machine gun had 300 to 400 rounds of locked, ready ammunition [Wallin 106, Zimm 268]. The machine guns got into action almost immediately, the 5-inch guns about four minutes later [Wallin 106-107]. Unfortunately, the 5-inchers were slow-firing guns, and their central director was not manned during the attack. In turn, the .50-caliber machine gun did not hit hard enough to take out aircraft dependably [Gannon 560-570].

However, the torpedo attack was brief, and AA fire did not become effective until near to its close, when 12 Kaga Kates made the final torpedo drops against battleship row [King 153]. Five of these B5N2s were shot down by anti-aircraft fire, and all but the first were damaged [Allen, King 153-154]. The five Kates lost to this final dozen torpedo attackers were the only B5N2s lost from the 144 that took part in the attack. If the battleships had been given even a little more warning, the results of the raid might have been different.

Kate with Torpedo Figure 15: Kate with Torpedo

Source: Imperial Japanese Navy via U.S. Navy.

In total, 40 Kates attacked with torpedoes. Figure 16 shows that 24 attacked from the east. The first 12 were from the Akagi, the final 12 from the Kaga [King 150]. This stream of Kates approached in line, several hundred meters apart. This delay between successive torpedo attackers gave individual torpedo bombers at least a brief time to pick individual targets. To give their torpedoes long enough runs to arm before striking, most B5N2s attacking from the east approached through the Southeast Loch (see Figure 16). Consequently, a large majority of the torpedoes launched at battleships were dropped against the Oklahoma and West Virginia, which lay directly in front of the loch. Only three Japanese torpedoes hit other battleships.

Type 97-3 Torpedo Attack at Pearl Harbor Figure 16: Type 97-3 Torpedo Attack at Pearl Harbor

Sources: Contemporary NASA satellite photo, Ray@Panko.com.

Sixteen other Type 97-3s attacked from the west. These Kates were from the Hiryū and Sōryū, and their hoped-for targets were Lexington and Saratoga, which they had been told the night before were not in port [Mori 1669].

Due to the mistaken identification of the Utah as a current battleship, six of the eight Kates from the Sōryū wasted their torpedoes on the former battleship and the ships around it [Fuchida 1952, Mori 1825ff, Panko Utah]. One more attacked Helena, which was a cruiser and therefore not a top-priority target [Mori 1809]. Only one swung around the island and attacked Battleship Row from the west. This Kate, piloted by Jūzō Mori, gave the California its second torpedo hit [Mori 1831ff], sealing its fate.

In turn, four of the Hiryū’s eight B5N2s also went after the Helena and Oglala, all missing these targets. The other four swung into the line of attacking Akagi and Kaga Kates to go after battleships. In total, then, out of 16 torpedoes, six were wasted on the Utah and the nearby Raleigh, and five more were wasted on Helena [Fuchida 1952] A mitigating factor in this squandering of so many torpedoes is that pilots were attacking into the low, early morning sun, so they had a difficult time seeing and identifying targets.

The High-Level Attack on Inboard Battleships in the First Wave
Figure 17 shows that some battleships were moored in pairs. One sat next to Ford Island, the other outboard. The inner ships — the Maryland, Tennessee, and Arizona — were safe from torpedoes.

Battleship Moorings Figure 17: Battleship Moorings

Source: National Archives Photo NH 50472, edited by Ray@Panko.com.

To attack the inner ships, a second swarm of 49 Kates flew over the backbone of Battleship Row, as Figure 18 shows. Each carried a single massive 800 kg (1,800 lb) bomb. They flew at almost 10,000 feet because dropping it from high altitude gave the bomb enough momentum to smash through the battleship’s armored deck. However, high-altitude bombing was not accurate. To compensate, the Type 97-3s flew in tight buntai chevrons of five, and all five dropped their bombs simultaneously. Even so, only eight of these bombs hit battleships [Zimm 232].

Type 97-3 High-Level Bombing Attack at Pearl Harbor Figure 18: Type 97-3 High-Level Bombing Attack at Pearl Harbor

Sources: Contemporary NASA satellite photo, Ray@Panko.com.

In addition, some of the bombs that did hit failed to fully explode. Their explosive was Type 91 [NTMJ Bombs 35], which is trinitroanisole [NTMJ Explosives 35]. Trinitroanisole is one of the least-sensitive explosives [NOAA]. It needed to be insensitive so that it would not explode immediately after slamming into the battleship’s deck at high speed. However, insensitivity made it difficult to detonate. In the same vein, the bomb was given two rear fuzes that would only fire if they sensed that the bomb had crashed through the deck. Two fuzes increased the chance that the bomb would detonate given the shock tolerance required in the bomb [NTMJ Bombs 35]. Trinitroanisole is highly toxic when it burns [NOAA], so if the bomb only partially detonated, it could be mistaken for a poison gas bomb.

The bombs that these Kates dropped in the first wave were Type 99 Number 80 Mark 5 special bombs [NMTJ Bombs p. 35]. The Type indicated that it was accepted in 1939. The Number was the bomb’s weight class in kilograms divided by 10, meaning the bomb was in the 800 kg class. (Its actual weight was 797 kg, or 1,757 lb.) Mark 5 indicates that it was a special armor-piercing bomb (not the fifth version of the bomb) [NMTJ Bombs p. 6]. The bomb was a special bomb only in the sense that it was not developed as part of the two main bomb programs — ordinary (anti-ship) bombs or land bombs. The bomb was fashioned from an obsolete 41 mm[1] shell from the battleship Nagato [Friedman 269]. For aerodynamics, the shell was tapered and fins were added.

Type 99 Number 80 Mark 5 Special Bomb Used in the First Wave Figure 19: Type 99 Number 80 Mark 5 Special Bomb Used in the First Wave

Source: U.S. Technical Mission to Japan [NTMJ Bombs p.35].

[1] Some writers say that the bomb was based on a 40 mm cannon shell, while others list the diameter as 41 mm. The confusion is due to a specific Japanese subterfuge. The original shells were created for the Nagato. Its guns had a diameter of 41 mm (16.1 in), but the Japanese reported it as only 40 mm (15.7 in) [Freidman 269]. They made this change on March 29, 1922, because the Washington Naval Treaty of 1922 barred guns larger than 16 inches (40.6 mm). Underreporting the shell size by a single millimeter allowed it to be reported as having an acceptable diameter. When it was converted into a bomb before the war, this fiction was maintained [NTMJ Bombs].

The bomb was a nearly solid steel dart designed to pierce armored decks using sheer mass and speed. It had room only for 22 kg (49 lb) of explosive charge [NTMJ Bombs p. 35]. Unless the bomb exploded in a powder magazine, it did little damage. Only one did, the bomb that obliterated Arizona.

The Second-Wave Attack on Airfields
An hour after the first wave began, 54 more B5N2s appeared in the sky. In the second wave, their prey was airfields, not ships. These Kates flew in chutais (squadrons) of nine aircraft.

[1] Some writers say that the bomb was based on a 40 mm cannon shell, while others list the diameter as 41 mm. The confusion is due to a specific Japanese subterfuge. The original shells were created for the Nagato. Its guns had a diameter of 41 mm (16.1 in), but the Japanese reported it as only 40 mm (15.7 in) [Freidman 269]. They made this change on March 29, 1922, because the Washington Naval Treaty of 1922 barred guns larger than 16 inches (40.6 mm). Underreporting the shell size by a single millimeter allowed it to be reported as having an acceptable diameter. When it was converted into a bomb before the war, this fiction was maintained [NTMJ Bombs].

Second-Wave Kate Targets
Combined Fleet Operations Order No. 2 [MHS 16] called for launching 54 Type 97-3s under the overall command of Lt. Cmdr. Shigekazu Shimazaki. These were divided into datais of 27 Kates each.

  • The Shōkaku B5N2s, led by Shimazaki, were to attack as a unit against Hickam Field. They did extensive damage there.
  • The Zuikaku Type 97-3s, commanded by Lt. Tatsuo Ichihara, were to hit three different targets: NAS Kaneohe, Ford Island in NAS Pearl Harbor, and Barbers Point.

The Zuikaku Kates certainly hit Kaneohe. Patrol Wing 1’s official report [PatWing1] said that two groups of nine attacked, although it referred to them as two-seat bombers. The facts that each dropped multiple bombs and dropped them in level flight, however, suggests that they were B5N2s. Wenger, Cressman, and Di Virgilio [1663-1672] corroborated the number of aircraft. The Kates at Kaneohe attacked at low level [PatWing1]. They needed to attack from below the clouds to aim, and the cloud base was low over Kaneohe that morning [Wenger, Cressman, Di Virgilio 1663-1672]. Fortunately for the Americans, the cumbersome Kate bombsight was inaccurate at low altitudes, so bombs had to be dropped by rough estimation [Wenger, Cressman, Di Virgilio 1678-1687].

If the remaining nine Kates in in Ichihara’s daitai had hit Ford Island, the loss of life probably would have been considerable. However, Ford Island did not receive noticeable bomb damage during the second wave, so it is not clear how the remaining nine Kates in Ichihara’s group actually proceeded.

Kate Targets for the Second Wave Figure 24: Kate Targets for the Second Wave

Kate Bomb Loads
Also according to Combined Fleet Operations Order No. 2 [MHS 16], each Kate was slated to carry one Type 98 #25 land bomb and six Type 97 #6 land bombs [MHS 16]. However, Figure 21 shows that some B5N2s, at least, carried two Type 98 250 kg bombs and no Type 97s. (This photograph was taken of B5N2s attacking Kaneohe.) Gunston [167] specifically said that 18 had two 250 kg bombs and that the remaining 36 had one 250 kg bomb and six 60 kg bombs. Unfortunately, Gunston did not give a source for his statement. It seems best to simply note the two bomb loadouts and say that both might have been used in the second-wave attack by Type 97-3s in Hawaii.

Kates Carrying Two 250 kg Land Bombs Attacking NAS Kaneohe Figure 21: Kates Carrying Two 250 kg Land Bombs Attacking NAS Kaneohe

Source: Wenger, Cressman, Di Virgilio.

Figure 21 and Figure 22 illustrate these two bombs. In Japanese terminology, land bombs were designed to be used against land targets. They were general purpose bombs, which primarily damaged through blast overpressure. However, they also produced shrapnel and heat damage, and were able to penetrate somewhat.

Type 98 #25 Land Bomb Figure 22: Type 98 #25 Land Bomb

Source: Naval Technical Mission to Japan [NTMJ Bombs p. 12].

Type 97 #6 Land Bomb Figure 23: Type 97 #6 Land Bomb

Source: Naval Technical Mission to Japan [NTMJ Bombs p. 8].

Table 1 has information about the explosive charges and penetration abilities of these bombs. Note that neither would have had difficulty penetrating roofs on hangars and other buildings. (A tour guide at Hickam erroneously said that these were armor-piercing bombs because they smashed through the roof in several buildings.) Their fuzes would be set to delay their detonation until they hit the floor or at least penetrated the building considerably.

Table 1: Bomb Characteristics for Kates in the Second Wave at Pearl Harbor
English

Bomb Weight (lb) Fill Percentage Explosive Charge (lb) Penetration of Reinforced Concrete (in)
Type 98 #25 534 40% 213 16
Type 97 #6 133 39% 52 8

Metric

Bomb Weight (kg) Fill Percentage Explosive Charge (kg) Penetration of Reinforced Concrete (cm)
Type 98 #25 242 40% 97 40
Type 97 #6 60 39% 24 20

Source: Naval Technical Mission to Japan [NTMJ Bombs 8 12].

Note that the Type 98 #25 land bomb had 213 lb of explosive charge. This was the largest explosive charge of any bomb the Japanese dropped on Pearl Harbor, including the big ship killer in the first wave. Whether a group of nine Kates carried one 250 kg land bomb and six 60 kg land bombs or two 250 kg land bombs, they would cause serious devastation.

Development

Arrested Development

During the 1930s, the Imperial Japanese Navy issued several requirements for carrier attack aircraft. Across most of these procurement cycles, the IJN failed to find anything acceptable, although it did order a few marginal aircraft. The Type 96 kankō — the Type 97’s immediate predecessor — came closest, carrying as much ordnance as the Kate. However, it had a top speed 50 kt slower than the B5N [Mori 696]. The Type 96 would be mincemeat in combat.

Nakajima and Mitsubishi Both Win

Japan finally held a successful prototype competition beginning in 1935 [Francillon 1995 412]. Nakajima and Mitsubishi competed. The Nakajima prototype was more advanced. It had hydraulically folding wings and combat Fowler flaps that made it very maneuverable [Francillon 1969 16]. It also had a variable-pitch propeller [Hawkins 5].

The IJN was concerned that these innovations might not be reliable in operational units, and tests did show problems with the power-folding wings [Francillon 1995 16, Hawkins 6], the Fowler flaps [Hawkins 6], and the variable-pitch propeller [Hawkins 6]. Most of these problems were fixed in the first prototype, but the IJN remained concerned about maintenance in operational use [Francillon 1995 413]. At the same time, the Nakajima prototype was clearly superior in performance to the Mitsubishi entry, which still had fixed landing gear [Francillon 1995 16]. The IJN decided to purchase the Nakajima machine.

To hedge its bet, the Imperial Japanese Navy also ordered some of the Mitsubishi kankōs [Francillon 1995 16]. However, the attractiveness of this backup strategy waned when Nakajima removed the power folding wings, Fowler flaps, and variable-pitch propeller in the second prototype [Francillon 1995 413]. This ameliorated reliability concerns. In the end, the IJN ordered only 115 Mitsubishi carrier attack aircraft. It ordered 1,149 Nakajimas.

Nakajima built 669 of the aircraft, while Aichi built 200 and Dai-Juichi Kaigun Kokusho built 280 [Francillon 1995 17]. This pattern of secondary production was common in World War II for both Japan and America. Nakajima production ended in 1941, allowing it to work on the attack aircraft’s successor [Francillon 1969 17].

The Type 97-1 Kankō (B5N1)

The initial production version was designated the Type 97-1 Carrier Attack Aircraft [Aireview staff 46, Burin Do, Eden 380, Francillon 1995, Hawkins 6, Mondey 215]. Its manufacturing project designation was B5N1. It was powered by a Hiraki 3 engine that produced 700 hp on takeoff and 970 hp at 3,000 m (9,485 ft) [Francillon 1995 413]. Its 9 cylinders were organized in a single row, making its diameter considerable. As Figure 24 shows, its cowling was wider than the fuselage.

Figure 25: Engine Cowling on the Type 97-1 and Type 97-3 Carrier Attack Bombers
Type 97-1 (B5N1) with Wider Cowling
Type 97-1 (B5N1) with Wider Cowling
Type 97-3 (B5N2) with Narrower Cowling
Type 97-3 (B5N2) with Narrower Cowling

Sources: Imperial Japanese Navy via U.S. Navy (Left) and U.S. Navy photo 80-G-427153 (Right).

In late 1938, the B5N1 entered combat in China [Hawkins 7]. At first, it was phenomenally successful. However, when Soviet pilots and aircraft began to intercept Japanese attack aircraft in China, losses began to mount [Eden 380, Francillon 1995 413]. When fighter protection was unavailable, Kates suffered heavy losses.

The Type 97-3 Kankō (B5N2)

To increase performance, the Japanese navy established the B5N2 manufacturing program to put a more powerful engine in the aircraft. Nakajima selected its own Sakae 11 two-row 14-cylinder air-cooled radial driving a three-blade constant-speed propeller [Francillon 1995 414]. This engine produced 1,000 hp at takeoff and 970 hp at 3,000 m (9,485 ft) [Francillon 1995 17]. Having two rows of cylinders, the Sakae engine had a smaller diameter than the one-row Hiraki 3 engine in the B5N1. This made the cowling smaller [Francillon 1995 414]. Cowling size is the easiest way to distinguish between the B5N1 and the B5N2 that followed, as Figure 24 shows.

The new version achieved operational status in December, 1939 [Eden 380, Francillon 1995 414, Hawkins 6]. It became the Type 97-3 Carrier Attack Aircraft [Aireview staff 46, Hawkins 7]. (The Mitsubishi kankō had been designated the Type 97-2 [Aireview staff 46, Hawkins 6-7]). Americans also code named it the Kate.

Disappointingly, the new engine produced no performance gain. The data in the Characteristics section of this paper explain why. Although the Sakae produced more than 40 percent more power at takeoff, it only produced 15 percent more at 3,000 m. In addition, it was 100 kg (221 lb) heavier. Cruising speed rose only from 138 kt (159 mph) to 140 kt (161).

However, the newer engine was more reliable, so production shifted to the B5N2 model anyway [Francillon 1995 414]. By the Pearl Harbor attack, Type 97-3s had replaced all Type 91-1s in front-line squadrons [Francillon 1995 414]. Some Type 97-1s were converted to trainers under the B5N1-K production designation [Francillon 1995 414].

Tenzan (B6N)

The small performance gain for the Type 97-3 convinced the IJN that it needed a completely new attack aircraft with more speed and range. This need was answered in 1939 [Eden], when Nakajima proposed a new carrier attack aircraft that looked like a scaled-up B5N with a larger tail surface in proportion to its body [Aireview staff 47]. Thanks to its engine, which was almost twice as powerful as the one in the B5N2, it would be able to fly 50 mph faster than the Type 97-3. It would also have 50 percent greater range [Aireview staff 47]. For added protection, its rear gunner would sit in a retractable turret, and it would also have a ventral machine gun to provide fire from attackers coming in below the aircraft [Aireview staff 47]. However, it still lacked a forward-facing machine gun.

The new aircraft was designated the Tenzan (Heavenly Mountain). It received a name instead of a type designation because the Japanese dropped the shiki designation system for operational aircraft in July 1943, replacing shiki designations with popular names [Francillon 1995, Mikesh 180-181]. Its manufacturing project code was B6N1.

B6N Tenzan
Figure 26: B6N Tenzan

B6N Tenzan after the war. Tested at the Technical Air Intelligence Center. U.S. Navy photograph posted to Wikipedia. https://commons.wikimedia.org/wiki/File:B6N_Taic.jpg.

The first Tenzan prototype flew in 1941; however, the Tenzan did not enter service until April 1944 [Aireview staff 47], a consequence of its long and difficult development process. A major problem initially was the use of Nakajima’s Mamori engine, which was powerful but heavy and vibrated severely [Aireview staff 47]. Replacing the Mamori with a Mitsubishi Kansei engine took considerable time, and engine problems proved to be only one of the Tenzan’s development issues [Aireview staff 47]. The B6N2 resulted from the replacement of the Mamori by the Mitsubishi Kasei engine. The B6N2 was the version that became operational. It was not until April 1944 that the B6N2 began to replace the 97-3 [Aireview staff 47]. As a result, the Type 97-3 continued to be used in front-line service, enduring severe losses.

Denouement

Although no longer viable for bombing and torpedo missions, the good range of the Type 97-3 made it acceptable for rear-area sea lane patrols to escort convoys [Francillon 1995]. To detect submarines, some Kates received a magnetic airborne detector (MAD)[1] capable of detecting submerged submarines by their magnetism [NTMJ MAD]. The MAD device used in the war was the Type 3 Model 1, indicating that it was accepted in 1943 [NTMJ MAD 1]. The MAD gear used a horizontal spool of wire wound 600 times [NTMJ MAD 12]. The magnetic field in this detector spool changed as the aircraft passed over a submarine, generating a tiny electrical flow in the wire loop. An oscilloscope-like display showed the changed flow.

Changes in aircraft pitch, roll, and yaw also induced a change in the magnetic field relative to the aircraft, so the loop was stabilized by a gyroscope to keep it dead level [NTMJ MAD 8-9].

[1] The MAD abbreviation today stands for “magnetic anomaly detection.”

In addition, the airplane’s electronics also induced small eddy currents in the aircraft fuselage, so these were measured by another wire loop and subtracted from the detector loop’s electrical readings [NTMJ MAD 10].

Despite intensive development efforts, the system could detect only a large submarine 120 m to 150 m (390 ft to 490 ft) below the aircraft and within 100 m (165 ft) laterally [NTMJ MAD 11]. Given the fact that the submarines being hunted were submerged, the vertical detection range was particularly disappointing. The Japanese navy concluded that this performance was inadequate, but having no other option, it implemented the MAD system [NTMJ MAD 7].

Given the very short range of MAD equipment, convoy protection required a group of six aircraft equipped with the MAD equipment to fly in a line abreast back and forth ahead of the convoy [USSBS 13]. Three thousand were ordered, but only 465 were delivered [NTMJ MAD 12], and only 90 to 100 were working at any time [NTMJ MAD 13]. Use was further limited by fuel shortages. Convoys often sailed without protection [NTMJ MAD 13]. Although our concern is the Kate, several aircraft types were used. In fact, only 50 of the sets were assigned to Kates [NTMJ MAD 15]. Overall, MAD-equipped aircraft were responsible for 11 submarine kills from August 1944 to July 1945. [NTMJ MAD 15]

Some Kates also carried a Type 3 Mark 6 Model 4 radar [FEAF 29]. These radars operated at a wavelength of 150 MHz (corresponding to a wavelength of 2 m.) This radar had a nominal range of 110 km (68 mi) for large ships. The range was much smaller for submarines. On convoy protection flights, radar-equipped aircraft would sweep behind the convoy, flying perpendicular to the convoy’s path. Given the radar’s long range, only a single aircraft needed to crisscross behind the convoy, and it could scan a considerable distance behind the convoy [USSBS 13]. The following figures illustrate the placement of these Yagi radar antennas on B5N2s.

Locations of Type 97-3 Radar Antennas Figure 27: Locations of Type 97-3 Radar Antennas

Source: CINCPAC.

Radar Antennas in Wing of Captured Type 97-3 Figure 28: Radar Antennas in Wing of Captured Type 97-3

Source: National Archives Photograph in Francillon [1969].

Location of Radar Antennas in Rear Fuselage of Captured Type 97-3 Figure 29: Location of Radar Antennas in Rear Fuselage of Captured Type 97-3

Source: National Archives Photograph in Francillon [1969].

By the end of the war, few Kates were left, thanks to combat losses, wear and tear, and their use in kamikaze attacks. Fearing rogue kamikaze attacks after the surrender, the United States destroyed nearly all remaining Japanese combat aircraft, including Kates. A few were taken for study, but no attempt was made to preserve them afterward.

Historiography Notes

The main historiography issue in this study is the question of the official designation for the version of the Kate that attacked Pearl Harbor — Type 97-3 or Type 97 Model 12. As discussed in the Appendix, Francillon [1995 52] said that the change from the one-digit model designation to the two-digit model designation occurred “in the late 1930s.” Mikesh [171] placed it later, “in late 1942 to 1943.” Neither cited a source. The staff of Aireview [46], a Japanese magazine, created a book on Japanese aircraft; it specifically stated in 1953 that the second version of the B5N was the Type 97-3. Although the preponderance of limited available evidence indicates that Type 97-3 was the correct shiki designation, agreement on the evidence is not unanimous. If future research clarifies this issue, I will update this study report.

Another frustrating issue was identifying which targets Lt. Ishihara’s 27 B5N2s hit in the second wave. The operational order for the fleet tasked his Kates to hit NAS Kaneohe, Ford Island, and Barbers Point. As discussed in the text, 18 did hit Kaneohe. However, bomb damage was not evident in the second wave at Ford Island, so nine of the Kates are unaccounted for. We have found no accounts written by crew members of second-wave Kates to cast light on this question. Again, if future research clarifies this issue, I will update this report.

A related question is the bomb load carried by B5N2s in the second wave. The operational order called for all to be loaded with a single 250 kg bomb and six 60 kg bombs. However, photographic evidence for NAS Kaneohe indicates that at least some carried two 250 kg bombs. Gunston [167] stated that 18 had two 250 kg bombs and the rest had one 250 kg bomb and six 60 kg bombs. Unfortunately, Gunston did not list a source for this statement. This is another area where I hope future research will bring clarification.

I have used primary or close-to-primary sources as much as I could. In particular, I have used several interview studies of Japanese technical specialists and senior officers. These interviews were conducted just after the war by U.S. forces. The Japanese officials proved to be surprisingly willing to give information despite likely resentment over the horrific destruction done to Japanese cities. In some cases, the cooperation appeared to reflect their desire to talk about achievements (and complain about problems). However, Wilkinson noted that there was often a more fundamental reason for cooperation. When he asked his Japanese contacts why they had given information so freely, he usually received the same answer: “Our emperor directed us to tell you everything.”

For the information on how Type 97-3s were used in the Pearl Harbor attack, we took information primarily from Japanese Monograph No. 97 [MHS]. This is a collection of operational orders and other official Japanese documents about the attack. Of special importance is Combined Fleet Operations Order No. 2, which details Japanese planning for the Pearl Harbor attacks, including targets, types and numbers of aircraft assigned to different parts of the attack, the weapons they carried, and the leaders of various groups in the attack. Information presented by authors who were present in the attack make it appear that the operational orders were followed generally during the attack, [Fuchida 2011, King, Mori]. However, several issues remained. As just noted, the most vexing were the Kate bomb loads on the second wave and what the nine aircraft that were slated to hit Ford Island actually did.

For general information about the Kate’s history, construction, and other matters, Francillon’s [1969 1995] books provide considerably more information than any other source. In fact, most other Western sources I have seen appear to take much of their information from his books, including directly copying text, usually without attribution. Several Japanese sources were particularly valuable because they looked at the Type 97 from Japan’s point of view.

My overall historiographical goal has been to present a comprehensive review of available information on the Nakajima Type 97 Carrier Attack Aircraft, resolving conflicts between statements in different sources where possible, and where impossible, pointing them out.

I have also added some analysis based on my background as a physicist, social scientist, information technology researcher, and business professor.

I would be grateful for any information that will help me correct any errors in this paper. I hope to offer revised versions of this paper in the future.

Characteristics

Category Type 97-1 Kankō B5N1 Kate Type 97-3 Kankō B5N2 Kate Tenzan Model 12 B6N2 Jill
Dimensions
Span 15.5 m (50’ 10”) Same 14.9 m (48 ft 10 in)
Length 10.3 m (33’ 10”) Same 10.9 m (35 ft 8 in)
Wing Area 37.7 m2 (406 ft2) Same 37.2 m2 (400.4 ft2)
Empty Weight 2,106 kg (4,634 lb) 2,279 kg (5,024 lb) 3,010 kg (6,636 lb)
Loaded Weight 3,700 kg (8,157 lb) 3,800 kg (8,378 lb) 5,200 kg (11,464 lb)
Maximum Weight 4,015 kg (8,852 lb) 4,100 kg (9,039 lb) 5,650 kg (12,456 lb)
Wing Loading 98.1 kg/m2 (20.1 lb/ft2) 100.8 kg/m2 (20.6 lb/ft2) 139.8 kg/m2 (28.6 lb/ft2)
Power and Performance
Engine 1 x Nakajima Hiraki 3 nine cylinder, one-row air-cooled radial driving a 3-blade constant-speed propeller. 1 x Nakajima Sakae 11 two-row, 14-cylinder air cooled radial driving a 3-blade constant-speed propeller. Mitsubishi Kasei 25 two row,14-cylinder air cooled radial driving a 4-blade constant-speed propeller.
Takeoff Engine Power 700 hp 1,000 hp 1,850 lb
Critical Altitude Engine Power 840 hp at 3,000 m (9,485 ft) 970 hp at 3,000 m (9,485 ft) 1,680 hp at 5,500 m (18,040 ft).
Power Loading 4.8 kg/hp (11.5 lb/hp) 3.8 kg/hp (8.4 lb/hp) 2.8 kg/hp (6.2 lb/hp)
Maximum Speed 199 kt at 2,000 m (229 mph at 6,500 ft) 204 kt at 3,600 m (235 mph at 11,810 ft) 260 kt at 4,900 m (289 mph at 16,075 ft)
Cruising Speed 138 kt at 2,000 m (159 mph at 6,500 ft) 140 kt at 3,000 m (161 mph at 9,845 ft) 180 kt at 4,000 m (207 mph at 13,125 ft)
Climb to 3,000 m: 7 min, 50 s 3,000 m: 7 min, 40 s 5,000 m (10 m 24 s)
Service Ceiling 7,400 m (24,280 ft) 8,260 m (27,100 ft) 9,040 m (29,660 ft)
Normal Range 679 mi 608 mi 1,085 mi
Weapons
Armament One flexible rear-facing 7.7 mm Type 92 machine gun. No forward-firing machine gun. Same One rear-facing 7.7 mm Type 92 machine gun in retractable turret, plus one ventral rearward facing Type 92 machine gun. No forward-firing machine gun.
Maximum Bomb Load 800 kg (1,764 lb) Same Same

Source: Francillon 1995, pp. 415-416 for the B5N and pp. 43-44 for the B6N2. B6N1 data is not listed in the table because the B6N2 was the version introduced.

Manufacturing Project Designation

Many American writers call the aircraft that attacked Pearl Harbor the B5N2. This designation is short, easy to remember, and similar to the U.S. Navy’s official operational designations at the time (F4F-3, etc.). However, “B5N2” was not used by pilots, air crews, or maintenance personnel because it was not an operational designation. It was a manufacturing project designation used by manufacturers and the ministries that financed these projects [Francillon 1995 51-52, Mikesh 175-179]. However, we will use it because of its widespread familiarity today and because of the fact that B5N2 is widely used in Japanese publications and models.

Breaking this designation down, B was the type of aircraft, in this case, a carrier attack aircraft. Other initial letters included A for fighters, D for dive bombers, and G for land-based carrier attack aircraft. The 5 meant that this was the fifth carrier attack bomber in the current series.[1] Finally, N indicated that it was created by Nakajima, and 2 meant that it was the second model of the aircraft.

Allied Code Name System

In turn, “Kate” came from the code name system used by the Allies, at least in the Pacific [Mikesh 10-24]. It was instigated by Captain Frank T. McCoy, aided by Technical Sergeant Francis M. “Fran” Williams [Mikesh 11-12]. McCoy was the “material officer” for the Allied Air Force, Southwest Pacific Area Headquarters. [Mikesh 11-12]. He and Williams developed their code name system because existing designations for Japanese aircraft were unworkable [Mikesh 12]. Their system was much simpler because it used easy-to-pronounce names—American boys’ names for fighters and girls’ names for bombers. Their system spread rapidly and was used extensively by U.S. and Allied operational forces during the war. However, the code name was not an official system when it began because it was not accepted at higher levels in the U.S. Army Air Forces [Avistar]. Fortunately, this unofficial status did not deter its use.

More pertinently, the Allied code name system did not appear until almost a year after Pearl Harbor. McCoy and Williams did not even arrive in Australia until 1942, and they did not develop the code name system immediately [Mikesh 12]. It was first promulgated in September, 1942 as Intelligence Information Memorandum No. 12: Japanese Air Services and Japanese Aircraft [Avistar]. Code names were not seen in combat reports until late that year and then only a few times [Avistar]. Therefore, the name “Kate” would not have been used by Allied forces at the time of the Pearl Harbor attack. However, I use it extensively because it was so widely used by operational forces on the Allied side during most the war and, more importantly, is so familiar today. The term Kate is also attractive for this document because it refers to both version of the aircraft.

Shiki Designation for Operational Use

The aircraft’s official IJN operational designation was the Type 97-3 Carrier Attack Aircraft (kyū-nana-shiki kanjō kōgeki-ki). The shiki (type) designation for this aircraft was Kyū-nana-shiki, which translates as Type (shiki) 97 (kyū-nana). Ninety-seven indicated that the Kate was accepted in Imperial Year 2597 (1937 in the Western Calendar) [Francillon 1995 413]. The shiki system was used operationally by the Imperial Japanese Navy [Francillon 1995 52-54], so we will use it extensively. Its major problem is that it is long. When the context was clear, therefore, the Japanese simply called the aircraft the Type 97 or the Type 97 kankō. I do the same.

Shiki Model Extensions

Aircraft are usually produced in several versions. In the 1930s, the IJA and IJN used the gata (model) system, in which the year designation was followed by a dash and a one-digit model number. The first version of the Kate was the Type 97-1 Carrier Attack Aircraft (The -1 meant Model 1) [Aireview staff 46, Burin Do, Eden 380, Francillon 1995, Hawkins 6, Mondey 215]. The version that attacked Pearl Harbor and was used through the rest of the war was the Type 97-3 [Aireview staff 46, Burin Do, Hawkins 7]. This was the only version in operational use at the beginning of the War in the Pacific [Francillon 1969 17]. Why was the second version not labeled the Type 97-2? As noted earlier, the IJN also accepted a Mitsubishi attack aircraft in 1997. This was labeled the Type 97-2 Carrier Attack Aircraft [Hawkins 6-7]. When the second version of the Nakajima kankō was created later, the B5N2, it was designated the Type 97-3 [Aireview staff 46, Hawkins 7].

This model system was changed over time. Instead of adding a single model number, the Type year was followed a two-digit model number, such as “Model 12” [Francillon 1995 53] This was pronounced “Model one two,” not “Model twelve.” It meant that this model used the 1st fuselage design but the 2nd engine. The revised model system was useful because fuselage designs and engines often changed at different times. In the revised model system, the initial aircraft would have been the Type 97 Model 11 Carrier Attack Aircraft. The version used at Pearl Harbor had a different engine, so it would have been the Type 97 Model 12 Carrier Attack Aircraft.

The key issue is when did the IJA and IJN convert from the one-digit model system to the two-digit model system? There are two major U. S. sources on the Japanese designation systems, and they disagree. Mikesh [171] says that the change occurred “Over a period from 1942 to 1943,” while Francillon [1995 52] says that the change took place “in the late thirties.” Both call the first model, the B5N1, the Type 97-1, with Mikesh [173] specifically calling it the Type 97-1, while Francillon [1995 413] expresses this as Type 97 Model 1. The disagreement comes with the B5N2. Mikesh [173] calls the second version of the Nakajima kankō (the B5N2) the Type 97-3 [173]. Francillon [1969 414] calls it the Type 97 Model 12. In the United Kingdom, Hawkins [6-7] also uses the Type 97-1 and -3 designations. He says that the -3 designation was “later” changed to Model 12 [7] but does not say when.

Terminology appears to vary by country.

  • S. sources tend to be follow Francillon [1995]. Although few list sources, expressions that Francillon uses, such as “in the late ‘30s,” appear frequently.
  • In the UK, terminology was consistent for the B5N1 in sources I located [Eden 380, Hawkins 5, Mondey 215]. All three call it the Model 1. For the B5N2, Hawkins [7] uses the Model 3 designation, while Eden and Mondey do not list model designations for the B5N2.
  • The two main Japanese language sources on the Kate that I have found were Aireview staff and Burin Do. Aireview staff call the B5N2 the Type 97 Number 3. Burin Do calls it the Type 97-3.
  • I was able to locate four Japanese-sourced plastic model kits for the B5N2. Table 2 shows that all four list the B5N2 as the Model 3, although they express it differently.

Table 2: Designations Used in Japanese Airplane Models of the B5N2

Manufacturer Manufacturing Project Designation Allied Code Name Operational Shiki Designation
Avioni-X B5N2 None Listed Model 3
Hasegawa B5N2 Kate Type 97-3
Marushin B5N Kate Type 97-3
Nichima B5N2 Kate Type 97-3

Given that only Francillon [1995] uses Model 12 for the B5N2, I chose to use Type 97-3 to refer to the second version of the Nakajima 1937 kankō.

[1] In the U.S. Navy designation system, in F4F-3, the 4F meant that it was the fourth fighter accepted from Grumman (F), not the fourth fighter in the current numbering series.

References

Aiken, David. (2001, December). Torpedoing Pearl Harbor, Military History, 46-53.

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Allen, Thomas B. (2001). Remember Pearl Harbor: American and Japanese Survivors Tell their Stories, Washington, DC: National Geographic Society.

Arakaki, Leatrice R. and Kuborn, John R. (1991). 7 December 1941: The Air Force Story, Hickam Air Force Base, Hawaii: Pacific Air Forces Office of History.

Avistar, Nakajima B5N Kate, Virtual Aircraft Museum. http://www.aviastar.org/air/japan/nakajima_b5n.php.

Beechy, Robert. (2013). Dai-Nippon Teikoku Kaigun Koukuu-tai (Imperial Japanese Navy Air Service) Aircraft Code Names & Designations, 1929-1945. http://hud607.fire.prohosting.com/uncommon/reference/japan/carrier_attack.html.

Bureau of Ships. (1947). USS Enterprise (CV6) War History, 7 December, 1941 to 15 August, 1945. https://www.history.navy.mil/research/library/online-reading-room/title-list-alphabetically/w/war-damage-reports/uss-enterprise-cv6-war-history-1941-1945.html.

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Figure 1: Semi-Monocoque Fuselage Construction
Figure 2: Crew Seating
Figure 3: Underside of Type 97-3 Wings
Figure 4: Overlapping Wings for Storage
Figure 5: Kate with Bomb Load. Probably with three 250-kg (550-lb.) bombs
Figure 6: Crew Seating
Figure 7: Pilot’s Seat
Figure 8: Type 92 Machine Gun (7.7 mm, .303 in)
Figure 9: Side View of Captured Type 97-3 Carrier Attack Bomber Showing an Unstowed Machine Gun
Figure 10: Kate Underside
Figure 11: Battleship Row, Carrier Berths, and Seaplane Base with PBY Patrol Bombers
Figure 12: Japanese Type 91 Modification 2 Torpedo with Anti-Roll Fins (Left) and Stabilizing Fins (Right) and
Figure 13: Rear Horizontal Rudder Effect when Torpedo is Upright on Water Entry (Unrolled)
Figure 14: Rear Horizontal Rudder Effect when Torpedo is Not Upright (Rolled) on Water Entry
Figure 15: Kate with Torpedo
Figure 16: Type 97-3 Torpedo Attack at Pearl Harbor
Figure 17: Battleship Moorings
Figure 18: Type 97-3 High-Level Bombing Attack at Pearl Harbor
Figure 19: Type 99 Number 80 Mark 5 Special Bomb Used in the First Wave
Figure 24: Kate Targets for the Second Wave
Figure 21: Kates Carrying Two 250 kg Land Bombs Attacking NAS Kaneohe
Figure 22: Type 98 #25 Land Bomb
Figure 23: Type 97 #6 Land Bomb
Figure 25: Engine Cowling on the Type 97-1 and Type 97-3 Carrier Attack Bombers
Figure 26: B6N Tenzan
Figure 27: Locations of Type 97-3 Radar Antennas
Figure 28: Radar Antennas in Wing of Captured Type 97-3
Figure 29: Location of Radar Antennas in Rear Fuselage of Captured Type 97-3