Design and development
The Ki-43 was designed by
Hideo Itokawa, who would later become famous as a pioneer of Japanese rocketry. The Ki-43 prototype was produced in response to a December 1937 specification for a successor to the popular fixed-gear
Nakajima Ki-27 Nate. The specification called for a top speed of 500 km/h (311 mph), a climb rate of 5,000 m (16,400 ft) in five minutes and a range of 800 km (500 mi). Maneuverability was to be at least as good as that of Ki-27.
When first flown in early January 1939,
 the Ki-43 prototype was a disappointment. Japanese test pilots complained that it was less maneuverable than the Ki-27 Nate and not much faster.
 In order to solve these problems, Nakajima produced a series of progressively modified prototypes through 1939 and 1940. These changes involved a major weight saving program, a slimmer fuselage with the tail surfaces moved further aft and a new canopy. Crucially, the 11th prototype introduced the unique differential "butterfly" maneuvering
Fowler flaps, which dramatically improved performance in tight turns. The 13th prototype combined all these changes, and tests of this aircraft resulted in an instruction for Nakajima to place the Ki-43 into production, the Ki-27 jigs being transferred to the
Mansyu factory at
Harbin in Japanese occupied
The Ki-43 (Oscar) was initially produced in November 1939, given the designation Ki-43-I. Deliveries from Nakajima's Ota factory commenced in February 1941. In addition to outstanding maneuverability, the Ki-43-I had an impressive rate of climb due to its light weight. Power was provided by the Nakajima Ha-25 engine turning a two-bladed, two-position variable-pitch metal propeller.
 Top speed was 495 km/h (308 mph) at 4,000 m (13,160 ft).
 The Ki-43 was equipped with two
cowling machine guns in various configurations, with either two 7.7 mm (.303 in)
Type 89 machine guns, one 12.7 mm (.50 in)
Ho-103 machine gun and one 7.7 mm (.303 in) gun, or two 12.7 mm (.50 in) Ho-103 guns; the aircraft was given various sub-designations to reflect these differences. The configuration that appears to have been most prevalent at the outset of the war was the first configuration with two 7.7 mm (.303 in)
Type 89 machine guns, while as the war progressed the heavier combinations gained popularity and the version with the heaviest armament was sometimes given the designation Ki-43-Ic.
 The Ho-103 was often loaded with explosive ammunition to increase target effect; its penetrative effect against later
armor appears to have been marginal.
Prototypes for the Ki-43-II flew in February 1942. The Ha-25 engine was upgraded with the 2-stage
supercharger, thus becoming the more powerful Nakajima Ha-115 engine, which was installed in a longer-chord cowling. The new engine turned a three-bladed propeller. The wing structure, which had suffered failures in the Ki-43-I, was strengthened and equipped with racks for drop tanks or bombs. The Ki-43-II was also fitted with 13 mm armor plate for the pilot's head and back, and the aircraft's fuel tanks were coated in rubber to form a crude self-sealing tank. This was later replaced by 3-layer rubber bladder, 8mm core with 2mm oil-proof lamination. The bladder has proven to be highly resistant against 7.7mm bullets, but was not much effective against larger calibers.
 The pilot also enjoyed a slightly taller canopy and a
reflector gunsight in place of the earlier telescopic gunsight.
 Nakajima commenced production of the Ki-43-II at its Ota factory in November 1942.
 Production was also started at the
Tachikawa Hikoki KK and the 1st Army Air Arsenal (Tachikawa Dai-Ichi Rikugun Kokusho), also at
Tachikawa. Although Tachikawa Hikoki successfully managed to enter into large-scale production of the Ki-43, the 1st Army Air Arsenal was less successful – hampered by a shortage of skilled workers, it was ordered to stop production after 49 Ki-43s were built.
 Nakajima eventually ceased production in mid-1944 in favor of the Ki-84, but the Tachikawa Hikoki continued to produce the Ki-43.
Tachikawa also produced the Ki-43-III, which utilized the more powerful Nakajima Army Type 1 Ha-115-II engine. Maximum speed increased to 358 mph.
 Tachikawa produced 2124 Ki-43-II and -III aircraft between April 1944 and the end of the war.
 Total production of all versions amounted to 5,919 aircraft.