The Mitsubishi Ki-51 'Sonia' was a very successful Japanese ground attack aircraft that remained in service throughout the Second World War. It was developed from the Mitsubishi Ki-30, a light bomber that made its maiden flight in February 1937 and entered service in 1938. Work on the Ki-51 began in December 1937 at the suggestion of Captain Yuzo Fujita. When the specification was revised in February 1938 it called for an aircraft with a top speed of 261mph, powered by a Mitsubishi Ha-26-II radial engine, armed with two forward firing and one flexible rear firing machine gun, and capable of carrying twelve 33lb or four 110lb bombs. The new aircraft also had to be manoeuvrable, and because it was expected to operate at low levels, to be unusually heavily armoured for a Japanese aircraft of this period.
The Ki-51 resembled a smaller version of the Ki-30. It used a similar fuselage and the same wing form, although the wings were mobbed from their mid-position on the Ki-30 to the base of the fuselage to reduce the length of the fixed undercarriage. The cockpit was shortened, bringing the two crew members closer together. The bombs were carried externally
The first two prototypes were completed in June and August 1939. Eleven service test aircraft followed by the end of the year - at this stage 6mm steel armour was added under the engine and cockpit.
It had originally been planned to produce two versions of the Ki-51 - the Army Type 99 Assault Plane and the Ki-51a Army Type 99 Tactical Reconnaissance Plane, carrying cameras in the rear cockpit. Instead of this the Army decided to give every Ki-51 the ability to carry cameras, and the aircraft could easily be swapped between roles in the field.
A total of 1,459 production aircraft were built by Mitsubishi and 913 by the Tachikawa Dai-Ichi Rikugen Kokusho (the Army's own arsenal). During the production run the 7.7mm wing guns were replaced by two 12.7mm machine guns, but otherwise the design remained unchanged. Production ended in July 1945.
The Ki-51 was used in a close support role in China and in every theatre where the Japanese Army fought during the Second World War. Although the Ki-51 lacked speed it was manoeuvrable, and unusually for a Japanese aircraft of the Second War, well protected. It was also easy to maintain and could operate from small airfields close to the front line. As a result it remained in use until the end of the war, and in production until July 1945. Only in the last few months of the war were the last surviving aircraft used for kamikaze missions, carrying one 551lb bomb under the fuselage.
Engine: One Mitsubishi Ha.26-II 14-cylinder radial engine
Power: 940hp at take-off, 950hp at 7,545ft
Wing span: 39ft 8in
Length: 30ft 2in
Height: 8ft 11in
Max Speed: 263mph at 9,840ft
Service Ceiling: 27,130ft
Range: 660 miles
Armament: One flexible rear-firing 7.7mm machine gun and two wing guns - 7.7mm on early production, 12.7mm on later production
Bomb-load: 441lb/ 200kg