Requirements were issued by the Imperial Japanese Army Air Force in 1937 for a replacement for the Ki-27, which was just entering squadron service. The new fighter was to have the same manoeuvrability as the older aircraft but with higher top speed and an extended range capability. Nakajima appointed Hideo Itokawa as the lead engineer on the Nakajima Ki-43 project.
The prototype of the new fighter was initially a disappointment in that it was unable to compete with the Ki-27 in a level dogfight, and was not much faster than the older model despite having a more powerful Nakajima Ha-115 engine and streamlined, retractable undercarriage. Improvements were made to later prototypes including a ‘Fowler’ type flap as trialled on the new Ki-44 fighter, which resulted in an improvement in agility sufficient to satisfy the IJAAF. Armament was initially the same twin 7.7mm machine guns as the Ki-27, but was later upgraded to 12.7mm weapons.
Problems became evident during the initial deployments as wing structures were found to be defective. Upgraded models were made available to the 59th and 64th that would take the Ki-43 into combat as the Pacific War began. The fighter dominated the skies over Malaya, Burma, and the Netherlands East Indies where it was often mistaken for the Zero due to their similar profiles. Indeed, the aircraft was often referred to as the “Army Zero”.
Later in the war Ki-43s were to be found in New Guinea, where they began to encounter Allied fighters with superior performance such as the P-38 Lightning and the F4U Corsair. In the hands of a competent pilot the Ki-43 could still be competitive, but as the standards of Japanese pilot training declined so did the effectiveness of the fighter. As the Allied counter offensive continued during 1944 the Ki-43 became a liability, and towards the end of the war it was primarily used in the kamikaze role.
In 1942 the Ki-43 was assigned the Allied Reporting Name “Oscar” in the Southwest Pacific and “Jim” in the China-Burma-India Theatre. When the duplication, was noticed, “Oscar” became the preferred reporting name.