Close this search box.

Yokosuka B4Y

The Yokosuka B4Y, also known as the Navy Type 96 Carrier Attack Bomber, was a torpedo bomber utilized by the Imperial Japanese Navy Air Service from 1936 to 1943.

This aircraft replaced the Mitsubishi B2M2 and was the final biplane bomber employed operationally by the Imperial Japanese Navy.

The Allied reporting name for this aircraft was “Jean”.

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

Aichi, Mitsubishi, and Nakajima each constructed a prototype in response to this requirement.

However, none of these aircraft were deemed satisfactory, and the service subsequently issued a new requirement, 9-Shi, in 1934 for a more capable aircraft to replace the outdated Yokosuka B3Y.

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

The Navy sought a torpedo bomber that could perform comparably to the Mitsubishi A5M monoplane fighter, and the result was a biplane with fixed landing gear and an all-metal structure with metal or fabric skin.

To expedite development and production, the B4Y utilized 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 equipped with the Nakajima Hikari 2 radial engine performed better than its competitors.

The B4Y had a crew of three, with the pilot occupying the open front cockpit and the other two crew members (navigator and radio operator/gunner) in the enclosed rear cockpit.

On 12 December 1937, three B4Y1s were involved in the Panay incident during a Japanese attack on the United States Navy gunboat Panay while it was anchored in the Yangtze River outside of Nanjing.

Although primarily used as a carrier-based aircraft, the B4Y1 was also occasionally employed as a land-based bomber.

In 1940, the Nakajima B5N replaced the B4Y1 as the primary carrier attack aircraft.

However, the B4Y1 remained in service as an advanced trainer and flew from Hōshō and Unyō until 1943.

Before its replacement, the B4Y1 saw action during the Second Sino-Japanese War and served at the Battle of Midway in June 1942, where eight of them were operated from Hōshō.

It was one of these planes from Hōshō that captured photographs of the burning Hiryū on 5 June 1942.


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,


500 kg (1,102 lb) of bombs.

Japanese Aircraft, 1910-1941-Robert Mikesh & Shorzoe Abe
The Mainichi Newspapers Co Ltd
Yushukan War Memorial Museum
Japanese Aircraft of the Pacific War-Rene Francillon,
Japanese Aircraft-John Stroud,
Japanese Military Aircraft Illustrated.


Share on facebook