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.
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