In analyzing the composition and tactics of the Japanese heavy bomber fleet in WWII, historians usually note two points: 1. the fierce rivalry between the Imperial Japanese Army and Navy for control over the Air Force which resulted in dissipation and ineffective deployment of resources in aircraft design and production as well as in battle: virtually all aircraft components were unnecessarily duplicated, from airframes and engines to machine gun rounds; 2. Japan’s lack of heavy long-range bombers, as they were understood by the western strategists, who were conducting an all-out aerial war by means of carpet bombing of industrial centers, military bases and residential areas in cities – the Japanese had no four-engine “flying fortresses,” capable of delivering several tons of ordnance over thousands of miles while flying at extremely high altitudes without fighter escort.
The only Japanese airplane that may truly be called a heavy bomber was the Kawanishi H8K («Emily») flying boat, of which only 167 units were manufactured. It had four 1680 hp engines and could carry 2 tons of ordnance to a distance of 7100 km. The four-engine Nakajima G5N and G8N bombers never passed beyond the prototype stage. The explanation for this state of affairs is quite simple: at the end of the 1930s Imperial Japan was preparing for a bliztkrieg. By that time, the Japanese army had been successfully fighting for a number of years in China and was now planning a major offensive in South-East Asia and Indochina, where its existing twin-engine bombers were expected to carry out bombing raids without encountering any opposition such as enemy fighter planes. Even warfare with the USSR would have required only minor modifications to the already existing warplanes, as the events at Halhin-Gol and Hasan Lake had demonstrated. Meanwhile, the Imperial Japanese Navy was preparing to destroy the British and American Pacific fleets, to provide support to Japanese landing operations on a number of Pacific islands thousand of miles apart from each other, and to protect extended maritime supply routes. As a result of such plans, extended range and the ability to attack enemy ships with torpedoes ranked as the most important criteria in the design specifications for a naval bomber. Thus, the Japanese Army and Navy planners decided to devote their efforts to the development of twin-motor medium-lift bombers almost entirely lacking any armor protection for the crew and vital aircraft systems. The Army was primarily interested in aircraft flight speed and the Navy in range.
H8K1 January, 1941 [2]
«Kawanishi» H8K2, 1943 [1]
By the end of 1941 the principal air strike aircraft in the Japanese Army was the Mitsubishi Ki-21-IIa bomber (allied code name – “Sally”) while land-based naval air groups included the outdated Mitsubishi G3M “Nell” as well as the latest Mitsubishi G4M1 “Betty” bombers. All these aircraft represented a significant advance for the Japanese Air Force when they were first fielded - their flight performance was equal to western counterparts, which they surpassed in landing and takeoff characteristics as well as range.
The Type 97 Army Heavy Bomber Model 1A Mitsubishi Ki-21-Ia was developed in 1936 in response to the Army’s need for a fast long-range bomber capable of striking remote targets in China and the Soviet Far East. The prototype, which flew for the first time on December 8, 1936 - only ten months after the development order was received, was a twin-motor, all-metal midwing monoplane with semi-monocoque stressed-skin airframe. It featured internal bomb bays at the wing roots, hydraulically operated flaps and landing gear partially retractable into pods. The strong and thin wing had three wing spars so that fuel tanks had to be located inside the fuselage. During the flight test program, which continued for another ten months, the aircraft empennage was expanded, the navigator's and gunner's cockpits modified, the location of machine guns changed and the size of the fuel tanks increased. The aircraft was powered by twin-row 14-cylinder radial Nakajima Ha-5 (Army Type 97) engines rated at 950 hp. Their design was similar to those of contemporary American engines, and the variable-pitch metal propellers were manufactured on license from Hamilton. Three small-caliber machine guns (WWI-era Vickers guns fed from 6 magazines with 69 rounds in each) were mounted in the nose, at the top and ventral machine gun stations. In November 1937 Ki-21 officially entered the Army arsenal after winning against a competing design – the Nakajima Ki-19. Production began in the spring of 1938 at the Mitsubishi plant in Nagoya, but demand was so high that beginning in August of the same year, the Ki-21-Ia went into production at the Mitsubishi plant in Ota and at a plant belonging to the competing firm Nakajima.
The Army wanted so many Ki-21-Ia aircraft simply because no other modern medium bomber design was available. In the Chinese campaign the Navy's Mitsubishi G3M bombers were used for the most part, a circumstance which undermined the Army’s attempt to establish control over the entire long-range bomber fleet. In the autumn of 1938, the Army's 60th air group, consisting of 30 Ki-21-Ia bombers, was formed and sent to China, to be joined in the spring of 1939 by the 58th and 61st air groups. Despite several hundred I-16 and I-15 fighters that China received from the Soviet Union by the autumn of 1938, combat losses of the Ki-21-Ia remained relatively low compared to those of the Navy G3M2 and Army Fiat Br.20. High flight speed (432 km/hr) and sturdy design were essential elements in this aircraft’s endurability.
In June and July 1939, 12 Ki-21-Ia from the 61st air group participated in the Nomongan incident (Halhin-Gol), where Soviet airplanes dominated and 6 Japanese machines were lost. The weakness of the self-defense weapons was clearly to blame, and in August 1939 a new improved «Model 1B» (Ki-21-Ib) started to roll off production lines. It was armed with two additional 7.7-mm Type 89 machine guns: one portable to fire from fuselage windows behind the wings and the second remotely controlled and mounted in the tail. On top of the fuselage was mounted a twin 7.7-mm machine gun with 10 magazines per barrel. The fuel tanks were lined with several layers of rubber. Bomb bay size as well as flap and empennage area were increased. This model was quickly followed by the next Model 1Ñ (Ki-21-Iñ), which featured a second side gun, optional fuel tank in the bomb bay, and external stations for ordnance. The increased aircraft weight required stronger landing gear. By February 1941 Mitsubishi had manufactured 120 Ki-21-Ibs and 160 Ki-21-Ics and Nakajima a total of 351 Ki-21-Is.
In November 1939 the military demanded an increase in the bomber's operating ceiling and cruise speed in the hopes that such modifications would protect it from enemy fighters. The first production Ki-21-Ic served as a prototype for the Ki-21-II. It was equipped with new larger-diameter propellers and new Mitsubishi Ha-101 Army Type 100 engines rated at 1500 hp at take off and 1540 hp at 2600 meters. The landing gear was now fully retractable into pods, and empennage area underwent further expansion. The flight testing was successfully completed in March 1940, and from December 1940 Model 2A (Ki-21-IIa) started to replace the earlier model on the production lines.
Ki-21-Ic, 1941
Ki-49-I, 1942
Meanwhile Nakajima was developing a new top-priority bomber project in accordance with the Army requirements of 1938. As the bomber was to operate without fighter escort, it required more powerful self-defense weapons, 15% higher cruise speed, and twice the range of the Ki-21-I. Applying the lessons learned in the case of the Ki-21 bomber, the firm designed a mid-wing monoplane with shorter wings to improve handling characteristics at low altitudes. Six self-sealing fuel tanks rested inside the fuselage and another two inside the wings. The Ki-49 was the first Japanese airplane to receive a tail turret and a 20-mm cannon in the top turret, in addition to the traditional nose, belly and side machine guns. Ten prototypes were featured in the test program, which took place from August 1939 until March 1941 when Type 100 Army Heavy Bomber Model 1 Nakajima Ki-49 officially entered the Army inventory. The 61st Air Group began to receive its first Ki-49 bombers in August 1941, and from February 1942 on the Ki-49 participated in bombing raids in China and, later, in New Britain and New Guinea. The allied code name for the aircraft was "Helen." The power of its two twin-row 1250 hp radial Nakajima Ha-41 engines was clearly inadequate - even with minimal bomb load, handling was poor and speed too low to escape enemy fighters. From August 1942, the Army started to receive first the improved Model 2A (Ki-49-IIa) version of the bomber equipped with 1450 hp Nakajima Ha-109 engines and then the Model 2B (Ki-49-IIb) version armed with 12.7-mm machine guns, which replaced the small-caliber weapons. However, the aircraft's flight performance remained poor. By January 1944, a total of 819 machines, representing all of the various modifications, had been manufactured--too few to replace all the Army's Ki-21 bombers.
Ki-21-II, 1943
Ki-21-IIb, 1943
At the outbreak of the great war, all of the Army's long-range bomber air groups targeted at Burma, Malaya and Singapore were flying Ki-21-IIa aircraft, with the exception of the 61st air group which was learning to fly a still newer Ki-49 bomber. During the first months of the war "Sally" was fairly successful in its support of land operations in Indochina but once allied fighters started to fly much more actively, the inherent weakness of its defensive weapons became only too obvious. Due to its small caliber and limited coverage, the upper machine gun was not very effective against dive attackers. By way of compensation, in 1942, following the issue of 590 Ki-21-IIà bombers, a new «Model 2B» (Ki-21-IIb) version went into production. In this version the glass "greenhouse" of the top gunner was replaced by a pedal-activated rotating turret armed with a 12.7-mm «Type 1» machine gun. By the end of that year, additional armor protection for the crew was introduced, and in this configuration the bomber was manufactured until September 1944 when the last Ki-21-IIb No. 688 was built.
One of the best know photos of Ki-21-IIb taken from an American bomber passing low over one of the airfields in New Guinea in 1944. Parachutes of extensively-used small-caliber fragmentation bombs are clearly visible.
The largest Japanese Army bomber in World War II, the Ki-21-II, flew from the first day of the war to its last, over a range extending from Northern China to Australia. On December 8, 1941 at 7 a.m. Japanese “Sally” bombers raided American military bases on the Lawson Island (the Philippines). In 1943-1944 the Ki-21-II bomber played an important role in the defense of New Guinea. By the end of 1944, all army air groups were reequipped with the Mitsubishi Ki-67 bomber, while the remaining airworthy Ki-21s were either converted to transports or assigned to special operations. The best known episode involving these converted and unarmed aircraft took place on May 24, 1945 when nine Ki-21-IIbs tried to land on Okinawa. The only airplane that succeeded made an emergency landing at Yontan air base with 12 kamikaze commandoes on board. Before being killed by American troops, the commandoes damaged several American B-29 Boeing «Superfortresses» and blew up fuel and ammunitions depots.
The Mitsubishi G3M, the principal Japanese naval bomber, was initially developed as a long-range maritime reconnaissance aircraft in accordance with specification “8-Si” of 1933. It should be noted that at that time the Vice Minister of the Japanese Navy and the chief of the Naval Aviation Bureau, which developed the airplane design specifications for Mitsubishi, was none other than Admiral Yamamoto Isoroku, who later became Commander in Chief of Japan's Combined Fleet. It was Yamamoto who insisted on awarding the development contract without any competition, for even at that time he was already advocating a long-range naval bomber force capable of supporting air carrier groups and attacking enemy heavy ships thousands of miles away from land bases. Yamamoto also wanted the Navy to recruit the talents of the main Japanese bomber aircraft specialist of that time--Mitsubishi's Toshio Honye, under whose supervision the company was manufacturing bombers for the Army under license from the German firm Junkers. When the Army began to require that all its aircraft be equipped with special naval equipment including torpedo stations, the Navy open hostilities in the bureaucratic war, the first shot of which was the “8-Si” specification.
The first reconnaissance prototype, designated Ka.9, flew in April 1934. It was an all-metal, midwing monoplane design equipped with two 500 hp Hiro engines. It also featured widely-placed flaps and a twin-fin tail typical of Junkers bombers. Parts of its surface were still made from corrugated metal, but for the first time in a Japanese-designed bomber, the landing gear was fully retractable into special pods. Western intelligence agents reported that this new Japanese G3M bomber was a copy of the German Junkers Ju-86. Indeed, the two aircraft were very similar in appearance, weight, dimensions, engine power and speed. But the Ju-86 did not fly for the first time until November 1934 and, more importantly, the Ka.9 had three times the range – up to 6000 km! Flight testing results were so encouraging that Navy Staff swiftly prepared design specifications for a naval strike bomber, whose main weapon was to be an air-dropped Type 91 450-mm torpedo carried externally. The bomber prototype, designated Ka.15, retained the Ka.9's wing and empennage, but the fuselage was completely reworked to accommodate five crew members and three extendable machine gun turrets. Ordnance was carried externally under the fuselage. The main problem facing the designers was the absence of a radial engine with the necessary power rating. Following the return of its engineers from company sponsored trips to the United States and Great Britain, Mitsubishi set to work on the new Kinsei engine. Although the first Ka.15 prototype was completed in June 1935, flight testing took place only in the spring of 1936 when the 910-hp Kinsei-3 was finally ready. The aircraft was immediately put into the inventory as Type 96 Naval Strike Bomber Model 11 G3M1 and at the end of 1936 a new Model 21 G3M2 went into production. It was equipped with Kinsei-41 engines rated at 1075 hp, as well as a Sperry-type autopilot and a radio compass, both manufactured under American licenses!
On August 14, 1937 24 G3M2 bombers took off from Formosa (Taiwan) for a bombing mission against Shanghai and Nankeen, but a thunderstorm forced the planes to make a more than 1000 km detour over the ocean. This unprecedented flight over a distance of more than 3200 km with full bomb load was immediately reported in the Japanese press. Foreign military experts dismissed these reports as propaganda. The Japanese press did not report however, that the same day another group of 18 G3M2 was intercepted by Chinese fighters over Hangzhou and turned back with a loss of six machines. In just one week the Japanese lost 54 aircraft! By the end of 1938 the tactic of bombing raids without fighter escort was abandoned, as the G3M burned like torches when enemy bullets hit the huge wings housing unprotected fuel tanks.
G3M2 «Model 21» armed with a torpedo and 250-kg bombs (left).
