need of effective interceptors to stop bombing raids over the Japanese mainland, so in October 1944 a decision was made to use the Mitsubishi Ha-112-II (Kinsei ["Venus"] 60 series), a 14-cylinder, two-row radial engine. The need for a new engine became urgent on 19 January 1945, when a bombing raid destroyed the Ha-140 production plant, leaving 275 otherwise complete Ki-61s engineless. The Mitsubishi Ha-112-II was lighter than the Ha-140 and developed the same power but with much greater reliability. Three Ki-61-II-KAIs were modified to carry this engine as prototypes. Chief engineer Takeo Doi with two other engineers redesigned the Ki-61 airframe to accept the new engine. Their solution was to use a second skin to form a fairing riveted to the fuselage to smooth out the airflow behind the cooling flaps and multiple exhaust stubs of the new engine cowling. As this engine was lighter, they were able to remove the lead counterweight in the tail that balanced out the heavier Ha-140 engine. The new model was flown for the first time on 1 February 1945. Without the need for the heavy coolant radiator and other fittings required for a liquid-cooled engine, the Ki-100 was lighter than the Ki-61-II, reducing the wing loading from 189 kg/m2 (38.8 lb/ft2) to 175 kg/m2 (35.8 lb/ft2). This had a positive effect on the flight characteristics, enhancing landing and takeoff qualities as well as improving manoeuvrability and reducing the turning radius. During March and April 1945, experienced instructors from the Akeno Army Flying School flew the Ki-100 in extensive tests against the Ki-84, which was the best JAAF fighter then in operational service. Their conclusions were that, given pilots of equal experience, the Ki-100 would always win in combat. The flight characteristics of the plane surpassed the Hiens in all but maximum speed, which was reduced by by the larger cross sectional area of the radial engine, and the model was ordered into production as the Goshikisen (Go=five; shiki=type; sentoki=fighter) or Army Fighter Type 5. The company's designation for it was Ki-100-I-Ko. All Ki-100-I-Ko were converted from existing Ki-61-II Kai and Ki-61-III airframes. The integral engine mount and cowling was cut off and a tubular steel engine mount was bolted to the firewall. Some redundant fittings from the liquid-cooled engine, such as the radiator shutter actuator, were left in place. The first 271 aircraft with the original faired rear fuselage were rolled out of