Many of the participating air powers of World War 2 (1939-1945) adopted twin-engine "heavy fighter" designs intended to provide greater range than that of a single-engine fighters of the period while proving capable of carrying greater armament loads. In this way, these designs could be used to counter enemy fighters one-on-one, escort allied bombers, or intercept incoming enemy bombers as required. The Germans had already found success with their twin-engine Messerschmitt Bf 110 and the British with their de Havilland DH.98 Mosquito. For the Empire of Japan, the Nakajima J1N1 "Gekko" (translating to "Moonlight") stood out as a fine twin engine heavy fighter example that went on to find several uses during the conflict. Its initial service tenure was as a three-man long-endurance reconnaissance platform until it became a short-term success as a night fighter when properly equipped and its crew reduced. By the end of the war, the role of the Gekko was reduced to bomb-laden suicide aircraft for kamikaze attacks against the Allies in the Pacific theater.
Origins of the Gekko were in a 1938 IJN (Imperial Japanese Navy) requirement for a twin-engine heavy fighter suitable for long endurance sorties as escorts for bomber aircraft. Success shown by the German Bf 110 design furthered thinking into a twin-engine heavy type for the needed operational ranges and armament suite. The Nakajima Aircraft Company went ahead with a prototype fitting Sakae series radial engines to low-wing monoplane wings along a smooth fuselage. Its crew numbered three and the undercarriage was the "tail dragger" arrangement. The tail unit utilized a single vertical tail fin and low-mounted horizontal planes. Armament became 1 x 20mm Type 99 cannon with 2 x 7.7mm Type 97 machine guns. A turret fitting an additional 4 x 7.7mm machine guns was also featured. The aircraft was designated J1N1.
In testing, the prototype failed to impress as an escort platform and modified for the reconnaissance role instead as the J1N1-C. Service entry, although slow, began in April 1942 and first actions against Allied forces was over the Solomon Islands to which the type was then given the codename of "Irving" by the enemy. The J1N1-C gave good service in its intended role for the interim as the Japanese still maintained the advantage in the Pacific theater.
However, fortunes for the Empire changed as night attacks on important positions increased. In 1943, Japanese Commander Yasuna Kozono of the 251st Kokutai had developed the idea of outfitting the J1N1 with heavier armament. This led to 2 x 20mm cannons being installed at the observer's cockpit, angled 30-degrees upwards to fire into the vulnerable underside of an unsuspecting enemy bomber. A J1N1-C was quickly modified in-the-field and testing in action successfully when two allied heavy bombers were downed. With more J1N1-C models modified in similar fashion, the J1N1-C KAI designation was born - these aircraft carrying an armament suite of 4 x 20mm Type 99 cannon in two upward- and two downward-firing mountings.
IJN authorities became aware of the successful modification and ordered Nakajima to quickly produce dedicated night fighter forms which begat the J1N1-S designation. This design saw its original crew of three reduced to two and the observer's position faired over. Production began as soon as possible in August of 1943 and ran until December of 1944.
In practice, the J1N1-S night fighters proved their worth against the lower-flying and slower-moving American bombers like the Consolidated B-24 Liberator. J1N1-S aircraft soon carried basic radar and search lights for night time scanning, Their downward-firing 20mm cannons were soon found to be rarely used in action and were appropriately deleted. With the introduction of the higher-flying Boeing B-29 Superfortress by the Americans, Gekko night fighters did not fare as well. Several kills of the large bombers were managed by Gekko crews but on the whole the type was outmatched. Even the addition of another 20mm cannon (to produce the J1N1-Sa mark) could not generate success.
With their wartime usefulness more or less over, and the crumbling war effort in Japan, the Gekko was fitted with bomb racks to employ a pair of 550 lb drop bombs to serve in the kamikaze role. These aircraft saw service into August 1945.
Other variants of the line included the J1N1-R which featured a 20mm Type 99 cannon in a dorsal turret and 1 x rear-firing 13mm Type 2 machine gun. These were later redesignated as J1N1-F.
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