Of all-metal construction with fabric-covered control surfaces, the B6N introduced no major aerodynamic improvements over the B5N, the increase in performance being achieved by the use of an engine offering some 80% more power than the Sakae 11 in the B5N2. In spite of a considerable increase in all-up weight, Matsumura was constrained by carrier stowage restrictions to use a wing with approximately the same span and area as that of the older aircraft.
The major external difference lay in the design of the vertical tail surfaces which on the B5N were swept forward to keep the aircraft's length within the limit of 36 ft 1.1 in (11.00 m) imposed by the size of the deck elevators. The Navy insisted on powering the aircraft with a Mitsubishi Kasei engine, but Matsumura elected to use the new 1,870 hp Nakajima Mamoru 11 fourteen-cylinder air-cooled radial as it had lower fuel consumption and a better growth potential. With this powerplant installed the first two prototypes were readied for flight trials in the spring of 1941.
The war against the Allies had not yet started and the Japanese Navy was confident that the B6N1 would be ready to replace the B5N2 in the near future. Unfortunately, early flight test reports indicated that the design suffered from serious engineering defects. The most urgent modification affected the vertical tail surfaces which had to be rotated 2º 10' to the left to correct directional stability problems stemming from the powerful torque of the four-blade propeller. After this modification the aircraft displayed markedly improved flying characteristics, but teething troubles, particularly with the Mamoru engine, slowed its development.
By the end of 1942 the Navy Experimental 14-Shi Carrier Attack Bomber was considered ready for carrier acceptance trials, but when the aircraft was tested aboard the Ryuho and the Zuikaku the mounting of its arrester hook proved weak, and the aircraft was involved in several landing mishaps. In early 1943, with the hook mounting strengthened, the B6N1, although requiring the use of RATOG (Rocket Assisted Takeoff Gear) units for takeoff at maximum gross weight, completed its carrier acceptance trials successfully.
After more than two years of testing, the type was finally accepted for production as the Navy Carrier Attack Bomber Tenzan (Heavenly Mountain) Model 11. The production aircraft introduced several modifications dictated by the result of the flight trials. The principal modifications included:
1. Replacement of the single exhaust stack, which produced excessive glare at night, by a series of
smaller exhaust units.
2. torpedo rack under the starboard fuselage side angled down 2º, and provision for torpedo stabilizing
tail plates to eliminate a tendency for the torpedo to bounce during low altitude release.
3. Strengthening of the main landing gear attachment and of the tailplane.
4. Addition of a flexible 0.303 in (7.7 mm) machinegun firing through a ventral tunnel.
Attempts were also made to replace the unprotected semi-integral fuel tanks with protected bag-type tanks, but this modification resulted in a 30% reduction in tank capacity and consequently the Navy decided to retain the original semi-integral tanks. In service the Tenzan performed satisfactorily, but its high landing speed and wing loading restricted its use to the larger carriers. During the battle off the Marianas, the first major engagement in which they participated, the Tenzans failed to achieve any significant results as the overwhelming superiority of the USN Hellcats deprived them of their escorting Reisens.
The Tenzan production program suffered a new blow when the Ministry of Munitions instructed Nakajima to cease manufacture of the Mamoru engine to enable concentration on the more widely used Sakae and Homare engines. Consequently Nakajima was forced to re-engine the Tenzan with the 1,850 hp Mitsubishi MK4T Kasei 25 originally specified by the Navy. The modifications required to install the new powerplant were comparatively small, and the program suffered only minor delays. After completing 135 Mamoru-powered B6N1s (including two prototype aircraft), Nakajima began delivering the Navy Carrier Attack Bomber B6N2 Tenzan Model 12 powered by the Kasei 25. These aircraft had also undergone a number of minor internal changes and were fitted with a non-retractable tailwheel, the tailwheel of the B6N1 being fully retractable.
Late production aircraft designated B6N2a Model 12A were armed with a flexible 0.511 in (13 mm) Type 2 machinegun in place of the dorsal 0.303 in (7.7 mm) Type 97 fitted to earlier machines. Like the earlier version, the B6N2 was known as Jill to the Allies, and aircraft of this type were active throughout the last two years of the war, being particularly aggressive during conventional and kamikaze attacks around Okinawa.
Two B6N2s, the 751st and 752nd production Tenzans, were modified to serve as prototypes for the B6N3 Model 13, a version powered by an 1,850 hp Mitsubishi MK4T-C Kasei 25c and featuring a strengthened undercarriage with larger wheels intended for operation from land bases with semi-prepared runways, as by then the Navy was practically without any aircraft carriers. The production of the B6N3 had not yet started when the war ended."