The Mitsubishi Ki-67 Hiryu (Flying Dragon) 'Peggy' was the best bomber to serve with the Imperial Japanese Army during the Second World War, but arrived too late to make any significant contribution to the Japanese war effort. Although it was designated as the Army Type 4 Heavy Bomber in Japanese service, it was nearer in capabilities to the American twin engined medium bombers. The aircraft was originally designed with a possible Japanese-Soviet conflict in mind, from specifications first drafted late in 1940, and given to Mitsubishi on 17 February 1941, to be developed as the Ki-67.
The Army specification called for an aircraft with an operating altitude of 13,125 to 22,965ft, a top speed of 342mph, capable of carrying a single 1,102lb to a target 435 miles from its base, with a normal crew of 6 to 8 and maximum crew of up to ten. The aircraft was to be armed with at least one 7.7mm machine gun in the nose, port and starboard positions and one 12.77m machine gun in dorsal and tail turrets. Power was to be provided by either two 1,450hp Mitsubishi Ha-101 radials, two 1,870hp Nakajima Ha-103s or two 1,900hp Mitsubishi Ha-104s.
Overall control of the project was assigned to Fumihiko Kawano, chief technical officer at Mitsubishi and the chief designer on the Ki-15, Ki-30 and Ki-51 reconnaissance and light bomber aircraft.
He appointed Hisanojo Ozawa as Chief Project Engineer. Ozawa was a specialist in designing Army heavy bombers, and had produced the Mitsubishi Ki-2-II, the first Army bomber to use retractable landing gear. He was also a co-designer of the Ki-21, and led the design team that produced the Ki-21-II.
Ozawa decided to use the 1,900hp Mitsubishi Ha-104 fan-cooled engine, a new design but one that was showing promise, and that began bench tests during 1940. The engines powered four-blade constant speed propellers. The aircraft itself was a slim, mid-winged monoplane. The wing and tail were similar to those of the Mitsubishi G4M1, but the new aircraft differed from standard Japanese practise in a number of ways. Ozawa designed it to be easy to build, using sub-assemblies to all production to be spread out, while the fuel and oil tanks were all self-sealing and armour was provided from the start.
The first prototype was completed in November 1942, and made its maiden flight on 27 December 1942. The second prototype was ready in February 1943 and the third in March. The new aircraft fell a little short of the required speed, but exceeded all of the other requirements, and Mitsubishi were given permission to build sixteen service test aircraft (nos.4 to 19). These aircraft carried more fuel, and heavier armament, with 12.7mm Type 1 machine guns in the nose and tail and one 20mm Ho-5 cannon in the dorsal turret, while the flush mounted port and starboard machine gun positions were replaced with blister mountings.
In December 1942 it was suggested that the Ki-67 be used as a torpedo bomber, and on 5 January 1943 Mitsubishi were ordered to fit torpedo racks on 100 aircraft. The 17th and 18th aircraft were used to test the new equipment, and the design was so successful that it was decided to add torpedo racks to every aircraft, starting with No.161. A number of torpedo-capable aircraft were operated by the Japanese Navy, as the 'Yasakuni', named after a shrine to an Unknown Soldier.
The design of the Ki-67 was frozen on 2 December 1943, when it was accepted as the Type 4 Heavy Bomber Model 1A Hiryu (Ki-67-Ia). This was similar to the service test aircraft, but with 12.7mm machine guns in the side positions.
The majority of Ki-67s were produced by Mitsubishi (606 aircraft). Kawasaki produced 91 aircraft at Gifu, Nippon Kokusai completed 29 Mitsubishi-built aircraft, and the 1st Army Air Arsenal at Tachikawa built a single aircraft. By the end of the 698 aircraft had been built. The first fully operation aircraft were delivered during the summer of 1944.
The Hiryu entered service too late to make any real contribution to the Japanese war effort, and after many of the best Army aviators had been lost over Rabaul and New Guinea. The aircraft made its combat debut with the Army's 7th and 98th Sentais and the Navy's 762nd Kokutai, operating as a torpedo bomber during the naval battles off Formosa in October 1944.
In the summer of 1944 Saipan fell into American hands, putting Tokyo within range of B-29 bomber raids. The Japanese Army reacted by preparing to launch bombing raids on the new American air fields on Saipan, forming the 2nd Independent Flying Unit at the Heavy Bomber Instructing Flight Division at Hamamatsu in July. The new unit began training to use its Ki-67s at long range over sea, a new mission for the Army, and by the time the Americans launched their first raids from Saipan in November 1944 the Japanese had already made two attacks on Isley Field, Saipan. The 2nd Independent Flying Unit was joined by the 110th Heavy Bomber Regiment, and the Ki-57 Hiryus of the two units became a familiar site over Saipan. The two army units merged into the 110th in December 1944, and were joined by the 7th Heavy Bomber Regiment.
The Japanese Army developed an elaborate plan for attacking Saipan. Mitsubishi Ki-46-IIIa 'Dinah' reconnaissance aircraft kept Saipan under constant observation. When the massive preparations for a B-29 raid began the Ki-67s would leave their bases in Japan and fly to Iwo Jima, where they would refuel and then continue on to Saipan. The attacks often came in just as the B-29s were preparing to take off on a raid, heavily laden with fuel and bombs.
Both sides suffered heavy losses during these raids - the Japanese lost aircraft to the intensive anti-aircraft fire on Saipan, to American fighter aircraft and on the long journey over a blacked-out Pacific. The costly raided ended on 19 February 1945 when the Americans landed on Iwo Jima, cutting the Japanese route in two. For the rest of the war it would be the Americans who used Iwo Jimo as a staging point.
The Ki-67 continued to make its presence felt to the end of the war, operating as a torpedo bomber against the Allied fleets at Okinawa and around Formosa, and as a heavy bomber over China.
The first production version of the Ki-67 was also the most numerous, accounting for aircraft 20 to 450.
From the 451st aircraft the single 12.7mm Type 1 Ho-103 machine gun in the tail was replaced with a twin 12.7mm mounting. 247 of the Ki-67-Ib were built before the end of the war.
The Ki-67-Ic was a design for a version of the Kiryu that was to have carried an increased 2,756lb bomb load. It was to have entered production with the 751st aircraft in the summer of 1945, but that target was never reached.
The Ki-67-I Kai was a suicide attack aircraft, produced by the Tachikawa Dai-Ichi Rikugun Kokusho. All of the turrets were removed and faired over, reducing the crew to three. A long rod extended from the nose to act as a detonator, and the aircraft could carry two 1,764lb bombs or a special 6,393lb explosive charge. The first aircraft were produced in September 1944, and twelve were ready to enter service by the end of October.
The Ki-67-II was to have been powered by two 2,400hp Mitsubishi Ha-214 radial engines. It would also have featured a stronger airframe and increased fuel capacity. One aircraft was half-completed at the end of the war, and production was to have started in the summer of 1946.
The Ki-69 was to have been an escort fighter variant of the K-67, designed to accompany formations of bombers. It was not built.
The Ki-97 was a design for a transport aircraft based on the K-67. It was not built.
The Ki-109 was a design for an interceptor fighter based on the Ki-67.
The Ki-112 was to have been a heavily armed fighter, probably designed to escort the suicide attack versions of the Ki-67 to their target. It was not completed.
Engine: Two Army Type 4 eighteen cylinder air-cooled radial engines (Mitsubishi Ha-104)
Power: 1,900hp at take-off, 1,810hp at 7,220ft, 1,610hp at 20,015ft
Wing span: 73ft 9 13/16in
Length: 61ft 4 7/342in
Height: 25ft 3 5/32in
Empty Weight: 19,068lb
Loaded Weight: 30,347lb
Max Speed: 334mph at 19,980ft
Cruising Speed: 249mph at 26,245
Climb to 19,685ft in 14min 30sec
Service Ceiling: 31,070ft
Range: normal 1,740 miles, maximum 2,360 miles
Armament: One 12.7mm Type 1 machine gun in nose, port and starboard positions, one 12.77m machine guns in tail turret for aircraft 20-450, two from aircraft 451; one 20mm Ho-5 cannon in dorsal turret
Bomb-load: Normal 1,102lb; Maximum 1,764lb; torpedo attack one 1,764lb or 2,359lb torpedo