The Mitsubishi Ki-109 with nose-mounted 75 mm Type 88 cannon.
In early 1943 the Mitsubishi Ki-67 heavy bomber then undergoing flight trials had proved that despite its size and weight it was fast and remarkably manoeuvrable. Consequently in November 1943, officers of the Rikugun Kokugijutsu Kenkyujo (Army Aerotechnical Research Institute) at Tachikawa suggested that the Ki-67 be used as the basis for a hunter-killer aircraft. The project received the designation Ki-109 and two versions were to be built: the Ki-109a, the killer was to mount in the rear fuselage two obliquely-firing 37 mm (1.46 in) Ho-203 cannon while the Ki-109b, the hunter, was to be equipped with radar and a 400 mm searchlight. However, soon thereafter, the project was re-directed at the instigation of Maj Sakamoto who suggested that a standard 75 mm (2.95 in) Type 88 anti-aircraft cannon be mounted in the nose of a standard Ki-67. It was hoped that with this large cannon the aircraft would be able to fire on the B-29s while staying well out of range of their defensive armament. As the Koku Hombu anticipated that, initially at least, B-29s would have to operated without fighter escort, the project was found sound and feasible and, accordingly, Mitsubishi were instructed in January 1944 to begin designing the aircraft, which retained the Ki-109 designation.
Modification of the Ki-67 to mount a 75 mm (2.95 in) Type 88 (Ho-401) cannon in the nose was entrusted to a team led by Engineer Ozawa and the first prototype was completed in August 1944, two months after the B-29s had made their first bombing raid over Japan. Except for its nose, in the lower part of which was mounted the Type 88 (Ho-401) cannon, the Ki-109 prototype was identical to the Ki-67 and retained the waist gun positions and dorsal and tail turrets of the bomber. Ground and inflight test firings of the heavy gun were affected by Maj Makiura of the Rikugun Kokugijutsu Kenkyujo and was sufficiently successful to warrant the placing of an initial order for 44 aircraft. The first twenty-four were each to be powered by two Mitsubishi Ha-104 radials rated at 1,900 hp for take-off, 1,810 hp at 2,200 m (7,220 ft) and 1,610 hp at 6,00 m (20,015 ft) but subsequent aircraft were to receive a pair of Mitsubishi Ha-104 Ru radials fitted with Ru-3 exhaust-driven turbosuperchargers and rated at 1,900 hp for take-off and 1,810 hp at 7,360 m (24,150 ft) to improve performance at the cruising altitude of the B-29s. These engines were actually tested on the second Ki-109 prototype, but no production aircraft were powered by Ha-104 Ru engines. Another attempt to im prove climbing speed was made when a solid propellent rocket battery was installed in the rear bomb-bay of the first prototype but this scheme was abandoned.
Starting with the third Ki-109, the dorsal turret and lateral blisters were dispensed with and no bomb-bay fitted. Fifteen shells were carried for the 75 mm (2.95 in) Type 88 cannon which was hand-loaded by the co-pilot, and the sole defensive armament consisted of a flexible 12.7 mm (0.5 in) Type 1 machine-gun in the tail turret. The rest of the airframe and the powerplant were identical to those of the Ki-67. Despite the lack of high-altitude performance the Ki-109 was pressed into service with the 107th Sentai but, by the time enough aircraft were on hand, the B-29s had switched to low-altitude night operations.
Manufacturer: Mitsubishi Jukogyo KK (Mitsubishi Heavy Industries Co Ltd).
Type: Twin-engined heavy interceptor.
Crew (4): Pilot, co-pilot and radio-operator in forward cabin and gunner in rear turret.
Powerplant: Two Army Type 4 (Mitsubishi Ha-104) eighteen-cylinder air-cooled radial engines. driving four-blade constant-speed metal propellers.
Armament: One forward-firing 75 mm (2.95 in) Type 88 cannon and flexible one 12.7 mm (0.5 in) Type 1 machine-gun in the tail turret.
Dimensions: Span 22.5 m (73 ft 9 13/16 in); length 17.95 m ( 58 ft 10 11/16 in); height 5.8 m (19 ft 1 1/32 in); wing area 63.85 sq m (708.801 sq ft).
Weights: Empty 7,424 kg (16,367 lb); loaded 10,800 kg (23,810 lb); wing loading 164 kg/sq m (33.6 lb/sq ft); power loading 2.8 kg/hp (6.3 lb/hp).
Performance: Maximum speed 550 km/h (342 mph) at 6,090 m (19,980 ft); range 2,200 km (1,367 miles).
Production: A total of 22 Ki-109s were built by Mitsubishi Jukogyo KK between August 1944 and March 1945.