Nakajima Type 1 Model 1 Army Fighter (Ki 43-I) Armament -- A Reassessment (part 1)

A Research Study by Richard L. Dunn (c) 2002, 2004


Popular works on World War Two history, such as Dr. Rene J. Francillon's tome Japanese Aircraft of the Pacific War, generally state that the Japanese Nakajima Type 1 Model 1 fighter (Ki 43-I), Hayabusa, was produced successively in three versions ko, otsu, and hei (Ki 43-IA, -IB, and -IC) armed, respectively with 2x7.7 mm machine guns; 1x7.7.mm machine gun and 1x12.7mm machine cannon; and 2x12.7 mm machine cannon. It is generally reported that the version with two 12.7 mm machine cannon (Ki 43-IC) was the major production version.

This paper presents evidence that while the twin 7.7mm version and twin 12.7mm version were introduced prior to the mixed armament version, the latter was introduced very early in the production run (prior to the outbreak of the Pacific War), was undoubtedly the major version of this aircraft to see action, and examples of operational aircraft with the alternative armaments are both relatively rare and may well have been retrofits. However, due to the ready inter-changeability of the two weapons types and absent direct evidence, the exact number of production types and retrofits could not be determined.


The Nakajima Company manufactured something over seven hundred Type 1 Model 1 fighters (Ki 43-I) for the Japanese Army Air Force [1]. This type was the predominant equipment of two Hiko Sentai (Flying Regiments hereafter abbreviated FR or Regiment) operating from Indo-China in the Malayan campaign during December 1941 [2]. During 1942 several additional Regiments formerly armed with the Type 97 fighter equipped with the Type 1 fighter. At the end of 1942 it was the predominant JAAF fighter. It was active in New Guinea, China, Burma, and the Netherlands East Indies. Flying against the British Commonwealth forces in Malaya, the RAF and American Volunteer Group in Burma, and the RAF, Dutch and Americans in the East Indies it was often identified as the "Zero." Later it was recognized as a separate type and given the Allied code name "Oscar."

Several publications on the Nakajima Ki 43-1 (or Oscar Mark 1) state that this aircraft came in three sub-types, which differed in their armament. These were the types ko, otsu, and hei, the Ki 43-IA (2x7.7mm), Ki 43-IB (1x7.7mm and 1x12.7mm), and Ki 43-IC (2x12.7mm). These same publications generally state that these models followed one another in serial fashion on the production line and that the twin 12.7mm version was the major production variant [3]. My reading of primary source documents made me suspicious that commonly held notions or the "received history" concerning this aircraft's armament found in many publications were wrong.

This paper presents research results which may substantially alter the common perception of the Ki 43-I's armament. However, the aircraft was flown in all the versions described. Furthermore comment will be reserved on whether the designations (a,b,c or ko, otsu, and hei) attributed to the various armament combinations were actually used during the service life of the aircraft (the author has found no evidence that this was the case). The direct and circumstantial evidence presented here is thought to be definitive. To the extent deemed otherwise, it should at least stimulate additional research and a new assessment of the popular version of this issue.


Both primary source documents and the general literature agree that the prototype and pre-production Ki 43-1's featured two armament combinations namely two 7.7mm machine guns or two 12.7mm "machine cannon " (in Japanese army parlance). The number 10 and 13 pre-production aircraft (serial numbers 4310 and 4313) carried two 12.7mm guns. The other prototype and pre-production aircraft carried two 7.7 mm guns [4].

After the capture of the Japanese airfield at Lae, New Guinea, Allied technical intelligence teams were quick to begin examination of the treasure trove of aircraft and documents found there. Among the papers captured there was a maintenance manual on the Ki 43, which, though undated, obviously applied to the earliest aircraft in the series. It stated:

"In this aircraft the type 89 fixed machine gun A, B, one each is installed in front of the pilot at the upper part of the fuselage. According to the situation the 12.7 mm aircraft machine cannon A, B, one each, can be installed in the position occupied by the MG" [5].

The notations A and B relate to the left and right ammunition feeds for the guns. The manual goes on to explain the relatively minor adjustments needed to install the machine cannon in lieu of the machine gun.


The manual captured at Lae and referred to in the preceding section establishes that early in its development the Ki 43-I came in two armament configurations (2x7.7mm and 2x12.7mm) and could easily be converted between the two configurations. An armorer's manual from the middle of 1942 describes "armament for airplanes used at the present time" [6]. For the Type 1 fighter only one armament configuration is mentioned - one "type 89 fixed machine gun" and one "Ho 103" (that is, 1x7.7mm and 1x12.7mm). This begs the question of how early the mixed armament was introduced. The answer seems to be from the time the very first unit was equipped with the aircraft.

One of the first two units equipped with the Ki 43-I was the 64th FR, which re-armed with the new aircraft from August to November 1941. Dr. Yasuho Izawa, a noted and respected aviation historian, has detailed the history of the 64th during this period in a rather detailed article. Dr. Izawa reports the early Ki 43's assigned to the 64th were equipped with one 7.7mm gun and one 12.7mm gun. He mentions no other armament configuration for Ki 43-Is of the 64ths from late 1941 to early 1943. This omission may be significant [7].

Dr. Izawa's report of mixed armament and early use of the 12.7mm machine cannon is bolstered by circumstantial evidence in post-war Japanese monographs which, while not primary sources themselves, were obviously written by authors with direct knowledge or access to primary sources. The first is the following comment in obvious reference to the Ki 43 equipped units:

"The stock-piles of the No.1 drop tanks and machine cannon ammunition were very small; therefore, it was feared that the long range fighters would not be able to accomplish their missions." [8].

The second set of data are two charts detailing the JAAF fuel and ammunition stocks in Indo-China at the outbreak of the war. Machine cannon (M.A.) ammunition was stockpiled at Duong Dong and Kompong Trach. These were the bases of the 64th and 59th FR, the only two units equipped with Ki 43s. None of the other aircraft at those bases was equipped with machine cannon and there were no such stockpiles at any other bases. As would be expected the bases also stockpiled 91 octane aviation gasoline and machine gun ammunition suitable for the Ki 43 [9].

Finally, records of ammunition expenditures for the 59th and 64th from December 8, 1941 to February 15, 1942, show that both units were expending both calibers of ammunition. In the case of the 59th their relative expenditure of both types was consistent with data from other units and logistics plans to be discussed later and indicative of the mixed armament. In the case of the 64th a higher proportion of 12.7mm ammunition was expended. A more detailed break down of ammunition expenditure, however, shows a higher use of machine cannon ammunition by both units in December 1941 followed by a decline [10]. This suggests these units may have had a number of twin 12.7mm armed fighters on strength at the beginning of the war with the numbers of such aircraft declining in succeeding months.

If the commonly accepted view described in the Introduction is correct, the "Ki 43-IC" with the twin 12.7mm armament was introduced sometime after the early phase of the war to which Dr. Izawa's comments and the data given above refer. The evidence is, however, to the contrary. In the units mentioned above the twin 12.7mm aircraft were present at the beginning of the war and then apparently declined in number. In late October 1942 the 50th and 64th FR, both equipped with Ki 43-I's, flew escort and strafing missions against Allied airfields in eastern India from Burma. After these raids the wrecks of four Ki 43's were discovered. The 64th FR reportedly lost one Ki 43 on October 25th and the other three losses were from the 50th FR on October 28th.

An Allied intelligence report in mid-November 1942 stated that four "Army 01 S/e fighters" had been located on the ground as a result of enemy raids on the Dinjan area at the end of October. No other type of fighter had been found. The report went on to state that despite reports to the contrary "the armament of this type has been found to be still only one 12.7mm cannon and one 7.7mm machine gun firing through the airscrew..." Ammunition belting for the 12.7mm cannon was described as almost entirely explosive and from reports of hits on Allied armor its penetrative power seemed poor [11].

One of the aircraft examined bore serial number 437 close to halfway through the Ki 43-I production run. Another Ki 43-I found in Assam after the October attacks bore serial number 618 (or roughly 2/3s through the production run).

From the comment in the intelligence report: "...the armament of this type has been found to be still only 1x12.7mm cannon and 1x7.7mm machine gun..." (emphasis supplied) it can be inferred that the intelligence officer considered this to be the previously encountered and expected armament of the Ki 43-I and thus exhibits a continuing state of affairs. The mixed armament thus appears to be the standard armament of the Oscar Mk. 1 in Southeast Asia as far as Allied intelligence is concerned.

continued in part 2