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. 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. 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. 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 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.
THE ORIGINAL ARMAMENT
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.
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.”
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.
ARMAMENT IN INITIAL OPERATIONS
The manual captured at Lae and referred to in the preceeding 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.” 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 suggests 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.
The first unit 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 is significant .
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.” (emphasis added).
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.
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 .
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 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 .
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.
ARMAMENT IN THE SOUTHEAST AREA
In the Autumn of 1942 things were not going well for the Japanese in their Southeast Area – primarily the Solomons and New Guinea. It was decided to send JAAF units to the area to bolster Japanese Navy air units already there. Among these was the 12th Hiko Dan (Flying Brigade, hereafter Brigade or FB) consisting of the 1st and 11th FR equipped with Ki 43-I’s. Before leaving these units were brought up to above normal strength in both aircraft and pilots.
The 11th FR transferred from Soerabaja to Truk by aircraft carrier in December 1942. Fifty seven Ki 43-I’s were flown from Truk to Rabaul on December 18th. As of December 31st the 11th had sixty one aircraft in the Southeast Area including three unserviceable aircraft on Truk. The 1st FR followed a month later with fifty nine Ki 43-I’s . These units are of particular interest since they are not only examples of Ki 43’s in the Southeast Area but brought with them aircraft formerly with other units in other areas.
In addition to the 120 Ki 43-I’s the 1st and 11th FRs brought with them, replacement aircraft accumulated at Truk by mid-March 1943 amounted to an additional seventy Type 1 fighters .
These 190 Ki 43-I’s represent over 25 percent of total production and with some thirty additional replacements previously forwarded from Truk constitute nearly thirty per cent of total production. Only relatively few of these aircraft were new. The 1st and 11th had first received the Ki 43 in mid-1942. As already noted they had received hand-me-downs from other units prior to transfer. Of the seventy Ki 43’s on Truk in March 1943 only thirteen were new. Others had previously been used by combat units and some had come from the Akeno training center. A number were damaged and had to be repaired on Truk or returned to Japan. All in all the equipment of these units was a good representative sample of Ki 43-I aircraft.
Without detailed records of the armament of each aircraft of the 1st and 11th FR we can still establish by strong circumstantial evidence that the mixed armament was in use. First, the Japanese 6th Air Division logistics plan for supplying ordnance to these units was based on about twice as many 7.7 rounds as 12.7 rounds. This proportion was exactly the same as for the Ki 61 but with one half the quantity. The Ki 61 was then armed with two 7.7mm guns and two 12.7mm guns or double the Ki 43-I’s armament. Second, all four Ki 43-I’s captured at Munda in mid-1943 were so equipped . Finally, we have a record of the ammunition expenditure of the 1st and 11th FRs during their first months of operations. The 11th expended 28,111 rounds of machine gun ammunition and 18,895 rounds of cannon shells. Figures for the 1st are 4,023 and 3,067 respectively . Actual expenditure of 12.7mm ammunition was about 40 per cent of the total for the two units. This compares to 30 per cent planned and compares closely to the 39 per cent for the 59th FR in early 1942.
To return to the Ki 43-I’s captured at Munda. Their armament was described as “one fixed 7.7 mm machine gun, type 89 improvement B, mounted on the top right side of cowling and synchronized. One fixed 12.7 mm machine gun, B, mounted on the top left of the cowling. Ammunition was described as standard ball and tracer for the 7.7mm with approximate capacity 500 rounds. A.P., H.E., and H.E. tracer for 12.7 mm with approximate capacity of 300 rounds. It was noted that it is possible to install two 12.7 mm without change in mounts .
These aircraft bore serial numbers 493, 685, 695, and 725. Their production dates range from June to mid-October 1942. These aircraft represent the 393d to 625th production aircraft out a little over 700 produced.
A representative list of 11th FR aircraft is found in the status report for No. 2 Chutai aircraft of that Regiment as of December 31, 1942 . Serial numbers range from No. 283 to 670 with 283 being an anomaly as no other serial numbers in the 200’s or 300’s are included. Other aircraft serial numbers were 414, 424, 450, 474, 579, 586, 634, 641, 649, 653, 658, 664, and 670. Number 646 had washed out in a force landing on December 29th (N.B. this was one of the Lae wrecks).
Ki. 43-I wrecks catalogued by Allied intelligence at Lae after its capture included: Nos. 239, 328, 397, 400, 426, 466, 520, 622, 646, 674, 805, and 810 . Many of these aircraft were completely wrecked and stripped. Available evidence does nothing to suggest other than that these aircraft had the “standard” mixed armament of one 7.7mm and one 12.7 mm. One of the last comments on the Oscar Mark I appearing in a Southwest Pacific intelligence report stated: “The armament of Oscar always consists of two synchronized guns…Normally one 7.7mm Vickers type gun is mounted in the left blast tube and one 12.7mm Browning type is mounted in the right blast tube.”[19a]
THE OTHER VARIATIONS
The twin 12.7mm version of the Ki. 43-I did get into field service. The Chinese captured one. This aircraft was captured on May 1, 1942 at Leizhou Bandao. In the Summer of 1942 both the 10th Independent Air Squadron and 24th FR operated the Ki 43 in China and the Flying Tigers had apparently first observed the type in operations over China early in July, however, neither unit was equipped with the type 1 fighter as early as May 1st and this particular aircraft belonged to a sprcial unit. An Army Forces Pacific Area intelligence report specifically states that the captured aircraft was armed with two fifty caliber machine guns (.5 inch=12.7mm). The aircraft was repaired and tested by the Chinese who assigned it serial number “P-5017.”
Given that two pre-production Ki 43-I’s were fitted with these weapons it does not seem surprising to confirm that this variant was flown in the combat theater. However in light of several published reports that this variant with two 12.7 mm guns (ostensibly the Ki 43-IC) was the major production type, it seems surprising to find so little evidence of its operational use. Moreover the aircraft described in the preceding paragraph may have been “one of a kind” as it was not assigned to an operational unit.
More surprising than the existence of one (possibly more) twin 12.7mm versions in an operational theater is evidence of late production versions equipped with twin 7.7mm guns. The wrecks of three Ki 43-I’s (s/ns 776, 804 and 808) were found at Cape Gloucester, New Britain in December 1943. Two were equipped with two 7.7mm machine guns and the third was in a demolished condition and its armament could not be determined.
It seems highly unlikely that late in its production life the Ki 43-I reverted back to its prototype armament configuration. A more likely explanation is that these aircraft were retrofitted in the field. As noted from earlier comments in this paper the conversion could be done rather simply without structural changes. Such an explanation begs the question, why?
While some Ki. 43-I’s from the 1st and 11th FR may have been repatriated to Japan for use as trainers or as a source of parts, a number were left behind in the Southeast Area. At least one was used by the the 14th Field Air Repair Depot for liaison purposes. Others probably went to the 13th FR that partially converted to the Ki 43 in addition to its Ki 45’s after suffering heavy losses at Wewak in August 1943. Some of the Ki 43-I’s may have been used as trainers. The two twin 7.7mm armed aircraft captured at Cape Gloucester were painted blue and no unit markings were reported. A similarly painted aircraft was later captured at Alexishafen, New Guinea. This unusual paint scheme may suggest that these aircraft were used for other than normal operational purposes. The 13th apparently received its first Ki 43-I’s in August and its first Ki 43-II’s in September .
If, as seems likely from the evidence, Ki 43-I’s were used for liaison and training purposes from August 1943, it would not be surprising to find their 12.7mm guns salvaged as spares or for use on other operational aircraft. Speculation? Yes, but more plausible than the production line turning out these lightly armed aircraft when 12.7mm cannon were more available than earlier .
EXPLANATION AND CONCLUSION
One nagging question remains. If the twin 12.7mm version was actually tested during the Ki 43’s prototype phase, why did the production model receive a mixed armament rather than two 12.7mm guns?
The explanation is suggested by information provided by Hiroshi Ichimura based on conversations with former 64th FR pilot Yoshita Yasuda. The early versions of the 12.7mm Ho-103 were simply not reliable and were subject to jamming. On occasion a shell would detonate in the barrel damaging the engine. The 64th may have lost as many as three aircraft to this cause during the Malayan campaign. The fix for this problem before more reliable versions of the gun were available was to mount iron plates on the blast tubes. These plates were in fact found on captured aircraft. The mixed armament was a compromise. The power of the relatively unreliable 12.7mm machine cannon was combined with the great reliability of the less powerful type 99 machine gun. 
The “received” version of the history of Ki 43-I as discussed in the Introduction is almost certainly wrong. If the Ki 43-I was originally placed in production with two 7.7mm machine guns, these early aircraft either did not go into action or were modified with one 12.7mm machine cannon prior to doing so. In early combat operations the 59th and 64th FR almost certainly operated aircraft with the mixed armament to the exclusion of the other versions. Not only does Dr. Izawa’s article state that this was the armament of the 64th FR Ki 43’s but ammunition expenditure data for the 59th FR confirms it for that unit. Three Japanese Monographs in addition to the one providing ammunition expenditure data provide general support for the mixed armament being used in early operations.
Crash intelligence regarding aircraft of the 50th and 64th FR indicates this armament was still in use by these units in Burma in October 1942.
The 1st and 11th FR took the Ki 43-I to the Southeast Area in late 1942 and early 1943. The evidence indicates that their aircraft (which represented a significant portion of the Ki 43-I fleet) were fitted with the mixed armament.
Limited examples of other versions of the Ki 43-I were found. However, even if these aircraft were produced with two 7.7mm or two 12.7mm guns and not modified in the field, their serial numbers are out of sequence with the commonly accepted history of this aircraft. The production sequence: A(2x7.7) –B(1x7.7 and 1x12.7) – C(2x12.7) clearly did not occur.
Based on the evidence marshaled in this study (which admittedly does not take into account all units equipped with this aircraft much less present direct evidence as to each aircraft) the main operational version of the Ki 43-I was equipped with one 7.7mm machine gun and one 12.7mm machine cannon. This version was in operation in Indo-China and Malaya early in the War; in Burma in October 1942; and, in the Southeast Area from late 1942 to mid-1943. A captured aircraft in China confirms the version with two 12.7mm machine cannon. While versions with two 7.7mm machine guns existed, they were likely retro-fitted aircraft relegated to non-combat roles.
19a. HQ AAF, SWPA Intelligence Summary
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