The Ki-30 was developed in response to a May 1936 Imperial Japanese Army specification to replace the Kawasaki Ki-3 light bomber with a completely indigenously designed and built aircraft. Mitsubishi and Kawasaki were requested to build two prototypes each by December 1936. The specification called for a top speed of 400-1NaN-1 at 3,000 m (9,840 ft); normal operating altitude from 2,000 m (6,560 ft) to 4,000 m (13,130 ft), the ability to climb to 3,000 m (9,840 ft) within eight minutes and an engine to be selected from the 634-1NaN-1 Mitsubishi Ha-6 radial, 615kW Nakajima Ha-5 radial, or 634-1NaN-1 Kawasaki Ha-9-IIb liquid-cooled inline engines, a normal bomb load of 300-1NaN-1 and a maximum of 450-1NaN-1, one forward-firing machine gun and one flexible rearward-firing machine gun, the ability to perform 60° dives for dive bombing, and a loaded weight less than 3300-1NaN-1.
The first Mitsubishi prototype flew on 28 February 1937 powered by a Mitsubishi Ha-6 radial. Originally, designed with a retractable main landing gear, wind tunnel tests indicated that the gain in speed was minimal due to the landing gear's extra weight and complexity and a fixed arrangement with "spatted" main wheels was chosen instead. The wing was mounted at a point above the line of the aircraft's belly in order to fully enclose the bomb bay within the fuselage. The pilot sat just above the leading edge of the wing, and the rear-gunner/radio-operator just behind the wing trailing edge, in a long "greenhouse" canopy which gave both crewmen excellent all-around vision. The Ha-6 engine drove a three-blade variable-pitch propeller.
A second prototype, fitted with the slightly more powerful Nakajima Ha-5 engine, was completed the same month. Although two months behind schedule and overweight, both prototypes met or exceeded every other requirement. The second prototype's top speed of 423km/h at 4,000 m (13,130 ft) led the Imperial Japanese Army Air Force to place an order for 16 service trials machines. These were delivered in January 1938 and the result of the trials was that the Army ordered the Ki-30 into production in March under the designation Army Type 97 Light bomber.
Mitsubishi built 618 production machines through April 1940, and the 1st Army Air Arsenal (Tachikawa Dai-Ichi Rikugun Kokusho) built 68 more by the time production ceased in September 1941. Including prototypes, a total of 704 Ki-30s were built.
The Ki-30s were first used in combat in Second Sino-Japanese War from spring 1938. It proved to be reliable in rough field operations, and highly effective while operating with fighter escort. This success continued in the early stages of the Pacific War, and the Ki-30s participated extensively in operations in the Philippines. However, once unescorted Ki-30s met Allied fighters, losses mounted rapidly and the type was soon withdrawn to second-line duties. By the end of 1942, most Ki-30s were relegated to a training role. Many aircraft were expended in kamikaze attacks towards the end of the war.
From late 1940, the Ki-30 was in service with the Royal Thai Air Force, and saw combat in January 1941 against the French in French Indochina in the French-Thai War. 24 aircraft were delivered, and were nicknamed Nagoya by the crews.[1] Additional Ki-30s were transferred from Japan in 1942.
