The torpedo bomber was born in the period preceding World War 1 (1914-1918) and had advanced considerably by the time of World War 2 (1939-1945) - though the basic attack concept remained the same. The type was a dedicated platform, its structure designed for the rigors of over-water flight as well as having the strength to carry and deliver a heavy torpedo payload against enemy surface ships. The biplanes of old had given way to all-metal monoplane winged designs by this time though some of the latter were still in service at the outbreak of the Second World War (as was the case with some of the entries concerning the British Royal Navy). The torpedo bomber gave warplanners the capability to launch coordinated attacks from aircraft carriers against enemy ships from the air - broadening torpedo attacks that once relied on warships outfitted with torpedo tubes or submarines lurking for short periods under the water.
The Nakajima Aircraft Company of Japan had been delivering naval-minded aircraft since 1927 (the A1N) and would produce a slew of designs leading up to, and during, the Grand War. In 1937, the prototype that would become the B5N torpedo and dive bomber (codenamed "Kate" by the Allies) flew for the first time. Service entry came a time later and 1,149 examples of this monoplane were produced. With prewar origins, a successor was developed by Nakajima in what became the B6N "Tenzan" ("Heavenly Mountain") and its own first flight was had during March of 1941 but a lengthy development phase meant that service entry was not until August of 1943. Some 1,268 examples of this design followed and the line was codenamed "Jill" by the Allies. The aircraft led a limited operational existence for air superiority had switched to the Allies as the war progressed towards it conclusion in 1945.
The B6N was designed to shore up the limitations discovered operationally with the earlier B5N series - namely performance and range. This led to the Imperial Japanese Navy (IJN) developing a new specification for a carrier-based attack platform of an all-modern design capable of hauling warloads up to 1,800 pounds out to 1,200 miles at cruising speeds of 230 miles per hour. Maximum speed was near the 300 mile-per-hour mark. The new Nakajima "Mamori 11" series 14-cylinder air-cooled radial engine would power the design which would include an operating crew of three made up of a pilot, radioman/gunner and bombardier (doubling as the flight's navigator). Initially it was expected that the Mitsubishi "Kasei" engine would be used - but Nakajima engineers successfully sold authorities on their in-house development instead.
This led to an aircraft of largely conventional design - a long, slender fuselage was required for the performance and hauling capability expected. A single engine, fitted to the nose, would power the airframe. The crew of three would be seated inline under a long-running, heavily-glazed canopy (greenhouse-style) assembly offering exceptional vision around the aircraft. A defensive position was fitted dorsally at the rear of the canopy section with a sole 7.7mm Type 92 machine gun on a trainable mounting being fitted. The torpedo load would be held externally under fuselage centerline. The wing mainplanes were low-mounted and fitted slightly ahead of midships. The tail unit was conventional with its single vertical fin and low-mounted horizontal planes. The undercarriage was of a "tail-dragger" arrangement with the main legs retractable under the wing elements. Dimensions conformed to the hangar elevators of Japanese carriers of the day and Fowler flaps were used to slow the aircraft's attack run and during landing actions on carrier decks. The Nakajima engine powered a four-bladed propeller unit.
The prototype went airborne on March 14th, 1941 though it was not an immediate success - the powerful engine produced a greater torque effect than expected and caused the aircraft to want to roll over consistently. This led to changes to the vertical tail unit but issues with the new engine were persistent as overheating and excessive vibration were noted through test flights. Modifications were had but served to also delay the program, and the aircraft was officially trialled on Japanese carrier decks during 1942. At the end of its evaluation phase, the engine proved suitable for operational service and a ventral 7.7mm machine gun position was added to protect the aircraft's vulnerable underside from trailing interceptors. This position was accessed by way of a tunnel and the machine gun was retractable to maintain aerodynamics when no in use.
In the early half of 1943, the design was contracted for serial production as the "B6N1". The aircraft would carry a single torpedo into battle, such was the standard of the day, and self-sealing fuel tanks were amazingly not a part of the finalized design - increasing operational ranges but making the aircraft highly susceptible to enemy fire of all kinds. Once production got underway and over 100 had been completed, it was decided to switch the temperamental engines with the originally-intended Mitsubishi Kasei 25 series of 1,850 horsepower. This led to a slight lengthening of the nose section to accommodate both the engine and a change to the airframe's natural Center-of-Gravity (CoG). These changes then produced the new designation of "B6N2" (Model 12).
Every third B6N2 was then outfitted with surface search radar equipment which gave a flight group the capability to find and mark its own targets. 1,131 B6N2 models were produced in all, marking it as the definitive model of the series. The B6N2a (Model 12A) featured a revised defensive armament arrangement which substituted the 7.7mm Type 92 system with the larger-caliber 13mm Type 2 heavy model to help increase firepower.
The B6N2 featured an overall length of 10.8 meters, wingspan of 15 meters and a height of 3.8 meters. Its empty listed weight was 6,635 pounds against a Maximum Take-Off Weight (MTOW) of 12,450 pounds. Maximum speed from the Kasei 25 radial engine (1,850 horsepower at take-off) was 300 miles per hour with cruising speeds reaching 205 miles per hour. Range was a useful 1,900 miles along with a service ceiling of 29,650 feet. Its warload included a 1,760 pound torpedo or the equivalent in conventional drop bombs.
With the war situation for Japan turning worse with each passing month, and its carrier attack force more-or-less neutralized by the Allied offensives, it was thought to create a land-based version of this aircraft which led to the "B6N3" (Model 13) designation. These would carry the Mitsubishi MK4T-C "Kasei 25c engine of 1,850 horsepower and feature a revised undercarriage for land-based service. However, the end of the war came too quickly for this variant and only two prototypes were ever completed.