Experimental 7-Shi Carrier Attack Aircraft (Nakajima B4N1, Y3B)

In April 1932, the Navy issued instructions to Mitsubishi and Nakajima to prototype a new carrier-based attack aircraft to replace the unpopular Type 89 Carrier Attacker. Nakajima, with Engineer Yoshida Takao as chief designer, developed the distinctive biplane Y3B equipped with the newly designed “Hikari” 73O engine. Two prototypes were built in 1933 and submitted for Navy evaluation. This aircraft featured a Gal-type upper wing and an inverted Gal-type lower wing, with the central section crossing in an X-shape relative to the slender fuselage. Furthermore, the landing gear struts were cantilevered from the lower wing's bend, representing an extremely bold structural design. The fuel tanks were exposed and removable, mounted on both sides of the fuselage, similar to those on the Nakajima E4N2 Reconnaissance Seaplane and the Nakajima A2N Carrier Fighter Model 2. At the time, the development of tri-seat carrier-based attack aircraft was a major challenge not only for Mitsubishi and Nakajima but also for Aichi and the Naval Technical Arsenal. Particularly, the selection of a reliable, high-horsepower engine proved difficult, preventing the establishment of a definitive design. Two prototypes were built in 1933 with the short designation B4N1*, and delivered to the Navy for evaluation but they failed to win Navy approval. Mitsubishi submitted its 7-Shi Carrier Attack Aircraft but it crashed on takeoff due to engine failure. Because of these failings, the Navy began with a new design competition for a 9- Shi Carrier Attack Aircraft.

Check this amazing scratch-built model here.

General characteristics
Crew: 3 (pilot, navigator/bomb aimer and gunner)
Length: 10 m (32 ft 10 in)
Wingspan: 14 m (45 ft 11 in)
Height: 3.80 m (12 ft 6 in)
Wing area: 50.0 m2 (538 sq ft)
Empty weight: 2,000 kg (4,409 lb)
Gross weight: 3,800 kg (8,378 lb)
Powerplant: 1 × Nakajima Hikari 2 9-cylinder radial engine, 520 kW (700 hp)
Propellers: 3-bladed

Performance
Maximum speed: 220 km/h (140 mph, 120 kn)
Stall speed: 93 km/h (58 mph, 50 kn)
Endurance: 6 hr
Service ceiling: 5,500 m (18,000 ft)
Time to altitude: 12 minutes to 3,000 m (9,800 ft)
Armament
Guns: 1 × flexibly mounted 7.7 mm machine gun
Bombs: 1 × 800 kg (1,760 lb) torpedo, or 1 × 800 kg bomb, or 2 × 250 kg bombs

*Francillon, Model Art #327 and others mention that the designation was "B4N1", only Mikesh-Abe mention that it was "B3N1". Considering that the Mitsubishi entry in the 7-Shi competition was designated B4M1 agreed by everyone including Mikesh-Abe and that the officially adopted type was the B4Y1 (also agreed by everyone), it is safe to conclude that the "B3N1" designation mentioned in the Mikesh-Abe book was a missprint. The Wiki entry needs correcting...