Yokosuka B4Y

The Yokosuka B4Y / Navy Type 96 Carrier Attack Bomber, carrier torpedo bomber was used by the Imperial Japanese Navy Air Service from 1936 to 1943.

The B4Y replaced the Mitsubishi B2M2 and was the last biplane bomber used operationally by the Imperial Japanese Navy.

In 1932, the Imperial Japanese Navy issued a requirement for a new carrier-borne attack aircraft. 

Aichi, Mitsubishi and Nakajima responded to this requirement, and each built a prototype.

None of these aircraft were deemed satisfactory, and the service thus issued in 1934 a new requirement, 9-Shi, for a more capable aircraft to replace the obsolescent Yokosuka B3Y.

The B4Y was designed by Sanae Kawasaki at the First Naval Air Technical Arsenal at Yokosuka.

Regarded only as an interim type, the Navy wanted a torpedo bomber offering performance comparable to the Mitsubishi A5M monoplane fighter.

The result was a biplane with fixed landing gear and an all-metal structure with metal or fabric skin.

To speed development and production, the B4Y utilised the wings from the Kawanishi E7K.

The B4Y1 was also the first Navy carrier attack aircraft to utilize an air-cooled engine, as the prototype that was equipped with the Nakajima Hikari 2 radial engine performed better than its competitors.

The crew of three occupied two cockpits.

The pilot in the open front cockpit and the other two crewmen, in the enclosed rear cockpit.


First Prototype 

559 kW (750 hp) Hiro Type 91 520 hp water-cooled W-12 driving a two-bladed propeller.

Second and Third Prototypes 

477 kW (640 hp) Nakajima Kotobuki 3 nine-cylinder air-cooled radial driving a two-bladed propeller.

Fourth and Fifth Prototype and Production Aircraft 

636 kW (853 hp) Nakajima Hikari 2 nine-cylinder air-cooled radial driving a two-bladed propeller.




10.15 m (33 ft 4 in)


15 m (49 ft 3 in)


4.36 m (14 ft 4 in)

Wing area

50 m2 (540 sq ft)

Empty weight

2,000 kg (4,409 lb)

Gross weight

3,600 kg (7,937 lb)


1 × Nakajima Hikari 2,

9-cylinder air-cooled radial piston engine,

630 kW (840 hp) for take-off

522 kW (700 hp) at 1,200 m (3,937 ft)


2-bladed fixed-pitch propeller


Maximum speed

278 km/h (173 mph, 150 kn)


1,573 km (977 mi, 849 nmi)

Service ceiling

6,000 m (20,000 ft)

Time to altitude

3,000 m (9,843 ft)

Wing loading

72 kg/m2 (15 lb/sq ft)


0.1749 kW/kg (0.1064 hp/lb)



1× flexibly mounted, rearward-firing 7.7 mm (0.303 in) Type 92 machine gun


1× 800 kg (1,764 lb) torpedo, or 500 kg (1,102 lb) of bombs.


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