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The Yokosuka B4Y Type 96 Carrier Attack Bomber 'Jean' was a torpedo bomber that the Allies erroneously believed was still in service in 1941, a mistake that played a part in their underestimating the threat from Japanese air power.
The B4Y was developed to replace the Yokosuka Bay Type 92 Carrier Attack Aircraft (B3Y), a disappointing design marred by an unreliable engine and stability problems. In 1934 the Japanese Navy asked Mitsubishi, Nakajima and their own design team at the First Naval Air Technical Arsenal (Dai-Ichi Kaigun Koku Gijitsusho) to produce aircraft to replace the B3Y. The arsenal was also known as the Kaigun Koku-sho, or Kusho for short, and the resulting aircraft is sometimes known as the Kusho B4Y and sometimes the Yokosuka B4Y after the location of the arsenal.
The B4Y was designed by Sanae Kawaski. It was a hybrid design, matching a new fuselage and tail to the wings of the Kawanishi E7K1. It had a all-metal structure, with a mix of light alloy and fabric coverings.
The first prototype, powered by a 600hp Hiro Type 91 liquid cooled engine, made its maiden flight towards the end of 1935. In 1936 four more prototypes were built, using two different engines. The second and third used a 640hp Nakajima Kotobuki 3 radial engine, while the fourth and fifth used a 840hp Nakajima Hikari 2 radial engine.
These prototypes then underwent competitive testing against the Mitsubishi Ka-12 and the Nakajima B4N1. The Hikari 2 powered B4Y1 won the contest, and in November 1936 was ordered into production as the Type 96 Carrier Attack Bomber.
The B4Y was a biplane with straight-edged un-swept wings. The upper wing has slight dihedral while the lower wing has a slight inverted-gull wing shape, with the fixed undercarriage at the lowest point of the wings. The radial engine had a smooth cowl. The three- man crew sat in two cockpits - the pilot in an open cockpit and the navigator and radio operator/ gunner in a rear cockpit with a canopy.
The B4Y was produced by three companies. Yokosuka only produced the five prototypes. Nakajima built 37 aircraft in 1937-38. Mitsubishi built 135 in 1937-38. The Dai-Juichi Kaigun Kokusho (11th Naval Air Arsenal at Hiro) produced 28 in 1938. The B4Y had a short front line career, and it had been withdrawn to a training role by 1940. The B4Y did see combat during the Second Sino-Japanese War.
By 1941 the B4Y had been replaced with front line combat units, but the Allies were unaware of this. The B4Y was thus given the credit for the sinking of HMS Prince of Wales and HMS Repulse in 1941. The B4Y had a maximum range of 978 statute miles. The aircraft that replaced it, the Nakajima B5N and Mitsubishi B5M, were much more potent, and had a longer maximum range, greatly extending the danger zone for Allied ships.
Engine: Nakajima Hikari 2 nine-cylinder air-cooled radial engine
Power: 840hp at take-off, 700hp at 3,940ft
Crew: 3 (pilot, navigator, radio operator/ gunner)
Span: 49ft 2 9.56in
Length: 33ft 3 19/32in
Height: 14ft 3 21/32in
Empty weight: 4,409lb
Loaded weight: 7,937lb
Max speed: 173mph
Climb Rate: 14min to 9,845ft
Service ceiling: 19,685ft
Range: 978 statute miles
Armament: One flexibly mounted 7.7mm Type 92 machine gun
Bomb load: One 1,765lb torpedo or 1,102lb of bombs