Specifications issued to Nakajima in 1939 for the Japanese Navy called for a 3-seat, carrier-based, torpedo bomber with a top speed of 464 kph (288 mph), a normal operating range of 1,853 km (1,151 miles), and a weapons load equal to the B5N.
Derived from a 1936 Navy specification, the B5N, Allied codename KATE, went into operational service in 1938 and saw considerable action in China. So thoroughly did U. S. Navy Hellcat pilots dominate the air that Japanese pilots flying the B6N sank no Allied ships and failed to inflict appreciable damage.
Japan attacked Pearl Harbor with three types of aircraft. Mitsubishi A6M Zero fighters controlled the air and strafed ground targets, Aichi D3A dive bombers bombed with pinpoint accuracy, and Nakajima B5N torpedo airplanes skimmed a few feet above the harbor to drop their deadly cargo. Derived from a 1936 Navy specification, the B5N, Allied codename KATE, went into operational service in 1938 and saw considerable action in China. It usually carried 800-kg (1,764 lb) torpedo or the equivalent load of bombs. Flat out, the KATE could fly at 378 kph (235 mph) as far as 1,992 km (1,237 miles) from base.
By 1939, the Japanese Navy had plans for a successor called the B6N. Specifications issued to Nakajima that year called for a 3-seat, carrier-based, torpedo bomber with a top speed of 464 kph (288 mph), a normal operating range of 1,853 km (1,151 miles), and a weapons load equal to the B5N. The B6N also had to operate from the same aircraft carriers as the KATE so it had to have the same overall dimensions. The size of the elevators used to raise the airplanes up from the hanger deck to the flight deck generated this size limit. The design team made no aerodynamic improvements to reduce drag and increase speed so the performance increased by installing a more powerful engine. The Navy wanted to use the Mitsubishi Kasei engine, but Nakajima chose its own design. The 14-cylinder, air-cooled, Mamoru 11 engine was more fuel-efficient than the Kasei power plant and it had better growth potential. The major external difference between the B5N and B6N was the vertical stabilizer on the new design. Nakajima had to sweep the vertical tail forward in order to meet the length constraint imposed by the size of carrier deck elevators.
Two B6N1 prototypes were ready in the spring of 1941 but early flight trials indicated serious problems. Nakajima had to redesign the tail to increase stability, and the engine caused trouble too. Not until the end of 1942 did the Navy judge the aircraft ready for carrier trials. Again, problems cropped up. The tail hook proved weak and often failed completely causing several accidents. The B6N weighed considerably more than the KATE and the new Mamoru engine could not heave the airplane off the flight deck and into the air without using rockets to boost the take-off. After additional modifications, the B6N1 entered production in early 1943. The Allies codenamed the bomber JILL but to the Japanese, she was the Tenzan or 'Heavenly Mountain.'
In service, the JILL performed satisfactorily but operations were restricted to the Navy's large fleet carriers because of a high landing speed and wing loading. The JILL made its combat debut during the battle for the Mariana Islands. So thoroughly did U. S. Navy Hellcat pilots dominate the air that Japanese pilots flying the B6N sank no Allied ships and failed to inflict appreciable damage. After delivering only 135 Tenzans, the Ministry of Munitions ordered Nakajima to cease building the Momaru engine and use only the Mitsubishi Kasei in the B6N. This new variant was designated B6N2 and Nakajima built more of them than any other model. Since Japan had lost most of her large carriers by mid-1944, B6N operations switched to land bases. They were particularly active during the Okinawa campaign and Navy pilots carried out conventional attacks with torpedoes and bombs, and 'toko' special-mission (suicide) attacks. Nakajima also completed two prototypes of a third variant, the B6N3, modified to operate exclusively from airfields, but the war ended before production began.
The NASM B6N2 is the last remaining Tenzan in the world. The U. S. Navy recovered it to the United States after the war along with about 145 Japanese aircraft shipped aboard three U. S. aircraft carriers. American technical experts tested evaluated these airplanes for a few years and eventually the airplane ended up at Naval Air Station Willow Grove, Pennsylvania, where it remained displayed outside until the NASM acquired it in 1981.