In September 1939, aircraft manufacturers Nakajima and Mitsubishi received an unofficial order to trial-manufacture the Raiden, the Japanese Navy's first interceptor (a fighter to undertake the air defence of land bases). It was finally decided, however, that the development and trial-manufacture of the Raiden should be conducted only by Mitsubishi. It was in April of 1940 that the plane was designated "14-Shi Interceptor (J2M1)" and official requirements were set out by the Navy. The requirements gave priority to climbing power, maximum speed and high altitude performance. Aircraft engines of those days that could meet all these requirements were only the air-cooled engine "Kasei" type 11 trial-manufactured by Mitsubishi for the Type 1 Attack Bomber (G4M) "Betty" and the liquid-cooled engine "Atsuta" trial-manufactured by Aichi. Because of the Navy's poor experience in liquid-cooled engines and Atsuta's insufficient reliability, the Navy was forced to employ the air-cooled engine Kasei which, having larger engine diameter and giving larger air resistance, was not best suited to be used in a fighter.