The first production model of the series was the Nakajima Ki-43-Ia. The Hayabusa had a Nakajima Ha-25 engine that was capable of propelling the Ki-43 at speeds of up to 495 km/h, with an internal fuel supply of 559 liters, supplemented by a pair of 204 liter drop tanks. The total armament of the Ki-43-Ia consisted of only two 7.7mm MGs mounted in the engine cowling. However, it could in fact carry up to two external bombs. The total wingspan of the Ki-43 was 11.5 meters.
The total length of the Ki-43 was about 11.5 meters and the total weight was 1,975 kg. The maximum range of the Ki-43 with drop tanks was about 1,762 kilometers, while service ceiling was about 11,734 meters.
The Ki-43 had a retractable undercarriage and was often regarded as simplistic to fly. It also had good reliability in the field, but was not very rugged as it would just fall apart with a few hits. Still, the fact that it was highly maneuverable can compensate for this somewhat and made it a fearsome opponent in combat. The key to this great maneuverability were special combat flaps added after the original pre-production Ki-43-Is scored poorly in this category - so low in fact that the design was nearly rejected.
The Ki-43 had many variants produced throughout its service life, many tried to correct the problem of the light armament which by 1943 was all but obsolete. The first of these variants is the Ki-43-Ib. This variant had a 7.7mm MG and a 12.7mm MG mounted in the engine cowling. Nothing else was added however. The next variant, the Ki-43-Ic was very similar in that the only change made was the addition of another 12.7mm MG, increasing the armament to two 12.7mm MGs mounted in the engine cowling.
The next variant, the Ki-43-IIa had much more in the way of improvements. Following the development of the Ki-43-II prototypes, the Ki-43-IIa has the same armament as the Ki-43-Ic, 13mm thick armor protection for the cockpit, a smaller wingspan, and finally the capability to carry up to 500 kg of bombs. The Ki-43-IIa also has self-sealing fuel tanks and a new 1,150 hp Nakajima Ha-115 Engine. The Ki-43-IIb featured new radio equipment, but was not changed in any other way and the Ki-43-II-KAI featured new exhaust stacks. Like with the earlier models, the Ki-43-III was simply a series of prototypes that led into the development of the Ki-43-IIIa. It's main difference from other planes in the series is the new and improved Nakajima Ha-115-II Engine. The final variant of the series was the Ki-43-IIIb developed by Tachikawa. Restricted to a pair of prototypes by the end of hostilities, the IIIb sported two 20mm autocannons in an attempt to correct the light armament. The Type I, Type Ib, Type II, Type III, etc. were all just designations for Ki-43s in IJA service.
Development of the Ki-43 began when the Nakajima company received a development contract at the beginning of 1938, just as the earlier Ki-27 was making it's operational début. The aircraft, codenamed "Oscar" by the allies, was first designed in early 1939. The specification called for an aircraft capable of destroying enemy bombers and escorting friendly aircraft. In addition, the new aircraft was required to possess superior combat performance to enemy interceptors, with maneuverability and cockpit visibility at least equal to that of the Ki-27, and speed, climb rate and range greater than those of all fighters known to be under development. The lack of self-sealing fuel tanks and pilot protection in the specification allowed designer Hideo Itokawa to produce a design that met the maneuverability requirement, with three prototypes being completed in early 1939. After brief manufacturer's trials, the three prototypes were passed to the JAAF, whose test pilots were critical of the type, stating that, compared to the Ki-27, the maneuverability of the Ki-43 left a lot to be desired, and the response to the controls was sluggish.
The aircraft saw combat in China and Japan's early military conquests such as Malaya and French Indochina. It also saw combat throughout the war and it was often misidentified for the A6M Rei-sen which shares a similar shape. Eventually, the Ki-43 scored the most Allied kills than any other Japanese aircraft made during the war. In total 5,751 were produced  and they were used in everything from bombing runs to kamikaze operations. The most produced version of the series was the Ki-43-IIa model and after the war, even the French used several Ki-43-IIIs captured in Indochina.
- Green, William. Famous Fighters of the Second World War. Purnell Book Services. 1975. Page 116
- Green, William - Fighters. Page 124
- Green William - Fighters Pages 116-117
- Green William - Fighters Page 117
- Green William - Fighters. Page 118