The designation Mitsubishi Ki-109 was used for two different attempts to produce an interceptor based on Ki-67 heavy bomber that would be capable of shooting down the new B-29 bombers. The first design, suggested in November 1943, was for a 'Killer-Hunter' team of two aircraft. The Ki-109a would have been the killer, armed with two obliquely mounted 37mm Ho-203 cannon, while the Ki-109b would have been the hunter, equipped with radar and a nose mounted 40cm searchlight. The second design was proposed by Major Hideo Sakamoto, the officer in charge of the Ki-67 evaluation programme. He suggested mounting an Army 75mm Type 88 anti-aircraft cannon in the nose of the Ki-67. This would reduce the speed and manoeuvrability of the aircraft, but allow it to operate outside the range of the B-29's guns, and was produced in the belief that the American bombers would be forced to operate without fighter cover. This second design was approved on 20 February 1944. The prototype Ki-109 retained the defensive guns of the Ki-67, but with a new nose and stronger fuselage. From the third aircraft the dorsal turret and lateral machine gun positions were removed. The prototype was completed in August 1944, two months after the first B-29s appeared over Japan, and production began later in the same year. Twenty two aircraft were produced using the Ha-104 engine as the Ki-109-I. They were to be followed by the Ki-109-II, using the 1,900hp turbo-supercharged Ha-104ru engine, but this version never entered production. The 107th Heavy Fighter Regiment was formed in November 1944, and received its aircraft in 1945. The Ki-109 failed to live up to expectations. The production Ki-109-I lacked the speed and rate of climb to catch the high flying B-29s on their early daylight raids, and despite a number of attempted interceptions never actually made contact with a B-29 formation. Once the Americans switched to low level night-time raids the Ki-109, which lacked radar, became completely useless, and the 107th Heavy Fighter Regiment was disbanded on 30 July 1945.