The Mitsubishi MK4 Kasei (Mars) fourteen-cylinder air-cooled radial engine was chosen to power the new fighter and, to reduce drag, was fitted with an extension shaft to permit a finely tapered cowling, an engine-driven fan drawing the air through a narrow annular intake. A laminar-flow airfoil section was selected for the wing, and so-called "combat" flaps were fitted to increase lift with the minimum of drag. An extremely shallow, curved wind screen was adopted for the cockpit, and forged components were used extensively in the structure.
Designated J2M1, the first prototype Raiden was powered by a 1,460 h.p. MK4C Kasei 13 engine driving a three-blade variable-pitch VDM airscrew, and was flown for the first time on March 20, 1942. Stability and controllability were praised by the manufacturer's test pilots, but it was complained that forward view was totally inadequate, that the curved wind screen distorted view for landing, and that the main landing gear members would not retract at speeds above 100 mph (161 kmh).
Throughout June and July the J2M1 was tested by the Navy at the Suzuka Naval Air Base, and the principal criticisms concerned the inadequate field of vision, the unsatisfactory operation of the airscrew pitch-change mechanism, and the fact that both speed and climb rate (357 mph at 19,670 ft, 575 kmh at 6,300 m and 7 min 48 sec to that altitude) were below those specified.
Three production J2M1 fighters were completed (the third is pictured above), but as the specified performance could not be attained with the MK4C Kasei 13, the fourth airframe was adapted to take the MK4R-A Kasei 23a which, with methanol-water injection, provided 1,820 hp for take off. With the more powerful engine the Raiden became the J2M2, and other changes included the provision of a four-blade airscrew, the introduction of new exhaust stacks which provided a certain amount of jet thrust, and a taller cockpit canopy embodying optically flat panels.
In October 1942 the Navy adopted the modified fighter as the J2M2 Raiden Interceptor, Model 11, but the test program ran anything but smoothly. Considerable effort was expended in eliminating vibration in the airscrew extension shaft, and at maximum power the Kasei 23a ran roughly, vibrated seriously and emitted a considerable amount of smoke. Engine vibration was somewhat reduced by modification of the engine mount shock absorbers, while the excessive smoke was eliminated by adjustments to the fuel and methanol-water injection systems. Vibration remained a problem, however, until it was eventually discovered that it could be suppressed by increasing the resilience of the engine-mount shock absorbers and the rigidity of the airscrew blades.
The second production J2M2 Raiden crashed on June 16, 1943 in temporarily inexplicable circumstances shortly after becoming airborne. The fighter had nosed down suddenly from some 70 ft (21 m), and a similar phenomenon was experienced with the tenth J2M2. Fortunately, the pilot of the latter aircraft immediately extended the landing gear and, upon landing, it was ascertained that the tail wheel shock strut had pressed against the elevator torque-tube lever as it retracted, jamming the control column forward!
In January 1944 the thirtieth 12M2 disintegrated over Toyohashi airfield and, although the cause of this accident was not satisfactorily explained, it was believed that violent vibration in the engine resulting from a broken attachment point had resulted in a secondary airframe failure. Another possibility was that the engine cowling had detached itself and hit the tail assembly. The engine attachment points were reinforced and the cowling fasteners strengthened, but other Raidens disintegrated in mid-air after these modifications had been made.
The 381st Air Corps at Toyohashi had received the first J2M2 Raiden fighters in December 1943 for pilot familiarization, and by this time the J2M2 had been supplanted by the J2M3 Raiden 21, 155 examples of the initial production model having been delivered. The J2M3 differed from its predecessor principally in armament. Whereas the initial production model had carried two fuselage-mounted 0.303 in (7.7 mm) machine guns and two wing-mounted 0.787 in (20 mm) Type 99 Model 1 cannon, the J2M3 had the 0.303 in (7.7 mm) guns eliminated and the wing cannon supplemented by a pair of faster-firing 0.787 in (20 mm) Type 99 Model 2 long-barrel cannon. Additionally it could carry two 66 lb (30 kg) or 132 lb (60 kg) bombs.
With the adoption of four Type 99 Model 2 cannon as standard, the designation was changed to J2M3a Raiden 21a. A new domed cockpit canopy was introduced on the production line in June 1944 in answer to pilots' continued complaints over poor vision in combat. With the new canopy the J2M3 and J2M3a became respectively the J2M6 and J2M6a Raiden 31 and 31a.
In an attempt to improve the Raiden's capabilities as a high-altitude interceptor, two prototypes were built with turbo-superchargers under the designation J2M4 Raiden 32, the first of these flying in August 1944. The four wing-mounted 0.787 in (20 mm) cannon were supplemented with two 0.787 in (20 mm) cannon mounted at an oblique angle in the aft fuselage, and maximum speed was 363 mph at 30,184 ft (584 kmh at 9,200 m), but the somewhat complex turbo-supercharger had faults which could not be quickly eliminated, and the J2M4 was abandoned in favor of the J2M5 Raiden 33.
First flown in May 1944, the J2M5 had the MK4U-A Kasei 26a engine with a mechanically-driven supercharger and a larger intake manifold. The J2M5 carried only two 0.787 in (20 mm) cannon which were mounted in the wings, but performance included maximum speeds of 381 mph at 21,588 ft (613 kmh at 6,580 m). and 375 mph at 26,247 ft (604 kmh at 8,000 m), and altitudes of 19,685 and 26,247 ft (6,000 and 8,000 m) were attained in 6 min 20 sec and 9 min 45 sec respectively.
The J2M5 was immediately placed in production at Mitsubishi's Suzuka plant and at the Koza Naval Air Arsenal, but despite the most strenuous efforts, only between thirty and forty fighters of this type were completed owing to the limited supplies of Kasei 26a engines, but these were immediately assigned to operational units, proving themselves among the most effective interceptors to be employed against the B-29 Superfortresses. The Kasei 26a was also installed in some Raiden 21 and 21a airframes which were redesignated J2M7 and J2M7a Raiden 23 and 23a.
The original Raiden production program for 1944 had called for the delivery of 3,600 aircraft and the attainment of a production rate of 500 aircraft per month by the end of that year. In fact, hardly more than 500 Raidens of all types had been completed by the end of the war. Plagued by teething troubles and production difficulties throughout its life, the Raiden was, nevertheless, a sound design basically.
The Raiden, which made its operational début during the Battle of the Marianas in September 1944 and fought in the defense of Japan's home islands until the end, enjoyed a steep climbing angle and rapid climb rate. Handling and control was good at all speeds from the stall to 325 mph (523 kmh), although the ailerons tended to be heavy above this speed. Stability was excellent, and although there was little stall warning the stalling characteristics were exceptional, recovery being very rapid with very little loss of altitude. There was no tendency to spin, and maneuverability was good.
The Raiden was allotted the allied reporting name Jack."
The following data relate to the J2M3.