Shortly before China-based B-29s initiated high-altitude daylight raids over Japan, it became ovcious to the Japanese Army leaders that an urgent development of an interceptor fighter with good performances at high altitude was necessary. In June 1943, the Army Aeronautical Research Institute studied the possibility of adapting the Hyakushiki Shiteir, which was one of the few operational aircraft in the Japanese Army to have the required altitude performance, to this type of operation. In May 1944, the project was approved and urgently carried out. To mount two 20mm machine guns on the nose, the forward fuselage fuel tank were replaced and nose canopy was modified to "stepped" nose housing. The aircraft with these modifications were called Hyakushiki Shitei III Kai Air Defence Fighter "Otsu" type. Further, in July 1944, the Army ordered to add a 37mm dorsal canon tofire forward and upward at an angle of 70 degrees. Aircraft with this addition was called "Otsu + Hei" type. Although most of them were flown by reconnaissance aircraft pilots with few dog figh experience, they made greater military gains against B29s proving the outstanding basic capability of the Hyakushiki Shitei.