The Mitsubishi Ki-21 (Army Type 97 Heavy Bomber) was produced by engineers Nakata and Ozawa in response to an operational specification issued by the Air Headquarters (Daihonei) of the JAAF on 15 February 1936. The original specification, issued in February 1936, had required an aircraft with an endurance of over five hours at an average cruising speed of 300 km/h (186 mph), able to climb to 3000 m (9840 ft) in eight minutes and reach a maximum 400 km/h (248 mph) at that height. Normal crew was to be four, with provision for two more gunners on missions where extensive air opposition was anticipated. Maximum bombload for short range missions was to be 1000 kg (2200 lb). Early production aircraft in fact met or exceeded all these criteria.

Mitsubishi Ki-21 Article

In its definitive form the Ki 21 was a midwing monoplane of all metal semi-monocoque construction, and the smoothly contoured fuselage was oval in section. It had a single fin and rudder, and the main under-carriage legs retracted forward into the two engine nacelles.

The first of eight prototypes and service trial aircraft flew on December 18, 1936. It had a square glassed in nose, dorsal gun turret and a ventral step for a lower defensive gun. Later prototypes adopted a long dorsal greenhouse accommodating the gunner’s position, a well contoured hemispherical glassed in nose and a smooth lower fuselage line which eliminated the ventral step. To improve directional stability the fin and rud¬der were redesigned and increased in area. The first prototype had two Mitsubishi Ha 6 825 hp radial engines, while the remaining prototypes and early production machines had 850 hp Nakajima Ha 5 Kais.

Before going into production the Ki 21 had survived a competition with its rival the Nakajima Ki 19. As a consolation a contract was awarded by the Koku Hombu (the army air headquarters) not only to Mitsubishi (432 Model Is built) but also to Nakajima (351 Model Is).

A year late in entering service, some of its technology, compared with that of contemporary foreign aircraft, was not as advanced as the designers Nakata and Ozawa had hoped. The gap left by the delay in delivery of Ki 21s had to be filled by importing Fiat B.R.20 bombers. The first production model being the Mitsubishi Ki-21-Ia (Army Type 97 Model 1A). Because of production bottlenecks it was not until the end of 1939 that Ki-21-la bombers equipped the first JAAF unit, the 60th Hikosentai (air regiment) based in China, the next unit to be equipped was the 61st Sentai.

Early lessons learned over China demonstrated lack of firepower and protection, and the Ki-21-Ib and Ki-21-Ic subvariants had extra armour, additional 7.7mm Type 89 machine-guns, more fuel and larger bomb-bays. The engines were 634kW Nakajima Ha-5 KAI radials.

By the time of the outbreak of war in December 1941, the majority of the Mitsubishi Ki-21-Ia, Ki-21-Ib and Ki-21-Ic bombers had been relegated to second-line duties, or to service as operational bomber trainers. Most army front line bomber units were equipped with the Model 2 version of Ki 21, powered by two 1450 hp / 1119kW Mitsubishi Ha 101 14 cylinder radials driving constant speed three bladed metal propellers, unlike the Ha 5 Kai radials of the earlier version which had variable pitch propellers. The larger and more powerful Ha 101s required larger nacelles, and these completely enclosed the retracted landing wheels in the earlier machines, the wheels were left partly exposed.

Production models in service in 1941 were the Ki-21-IIa (Army Type 97 Heavy Bomber Model 2A), and the Ki-21-IIb which had a pedal-operated dorsal turret with one 12.7mm Type 1 heavy machine gun. Three sentais remained in Japan, Korea and in Manchuria when the Japanese high command went to war in South East Asia. For operations over the Philippines the JAAF’s 5th Air Group, based in Formosa, mustered the 14th and 62nd Hikosentais; these went into action early on the morning of 8 December 1941 striking at Aparri, Tuguegarao, Vigan and other targets in Luzon. Mitsubishi Ki-21s of the 3rd Air Group, based in French Indo-China, were earmarked for bombing strikes against Siam (Thailand) and Malaya: units were the 12th, 60th and 98th Hikosentais. These smashed RAF and RAAF facilities at Alor Star, Sungei Patani and Butterworth, being escorted by Nakajima Ki-27 and Ki-43 fighters. Only over Rangoon over December 1941 and January 1942 did the Ki-21s, codenamed ‘Sally’, suffer heavy casualties.

At its peak the Ki 21 was supplied to 11 first line Sentais and played a considerable part in the Pacific air war. Towards the end of its career lack of Ki 49 and Ki 67 replacements forced the retention of the Ki¬-21 in the first line when it was obsolete and suffering heavy losses. It took part in a number of outstanding actions, perhaps the most notable of which was the suicide mission by nine Model 2s of the 3rd Dokuritsu Hikotai (independent air wing). These were to land on the US occupied airfield at Yontan on Okinawa in the summer of 1945 to launch commando style attacks on airfield installations and aircraft. Only one aircraft survived to deliver its troops, but considerable damage was inflicted before they were wiped out.

The original Ki 21 defensive armament of single flexibly mounted 7.7 mm (0.303 in) Type 89 machine guns mounted in nose, dorsal and ventral positions proved barely adequate even for operations against the poorly equipped Chinese air force, and the Ki 21 Ib had an additional gun in a remotely operated tail stinger position plus another which could be fired from lateral positions on either side of the fuselage. The Ki 21 Ic and IIa had two beam guns while the Ki 21 IIb (688 of which were built between 1942 1944) had a remotely controlled tail stinger position for a 7.7 mm (0.303 in) machine gun, and the dorsal greenhouse replaced by a large conical turret mounting a single 12.7 mm (0.54in) Type 1 machine gun. This latter change greatly altered the appearance of the Ki 21 IIb and for a period it was thought by the Allies to be a new bomber type and given a different codename, Gwen, which was soon dropped when the new version was properly identified.

The bombload of all Ki 21 versions remained the same: a normal load of 750 kg (1653 lb) and a maximum of 1000 kg (2205 lb). The maximum range was 2700 km (1680 miles).

Some Ki 21 Ia bombers were modified during 1940 and put into service as MC 21 freight transports. Formally they passed into civil use as they were operated by the Dai Nihon Koku KK (Greater Japan Airlines) on routes between Japan, China and Manchuria, but all these flights were in fact military contract work, carrying army supplies (or personnel). Although all armament and other equipment appropriate to the bombing role had been removed, the aircraft were externally identical to the bombers, until a number were modified by the fitting of a ‘solid’ nose.

The Ki.21 was coded ‘Jane’ after Gen. MacArthur’s wife but when this came to his attention at the General’s request the code was changed to ‘Sally’,

Code named Sally, a modified model with a smooth dorsal line and a gun turret in place of the ‘greenhouse’ led spotters to believe a new type was entering service. The name ‘Gwen’ was assigned, but soon dropped in favour of ‘Sally 3’ when it was identified as a model variation.

Production of the Ki 21 ceased in September 1944, with the 2064th aircraft.

Ki-21 (Army Type 97 Heavy Bomber) prototype
Engines: 2 x Mitsubishi Ha 6 825 hp radial or 850 hp Nakajima Ha 5 Kais.
Propellers: variable ¬pitch
Span: 22.5m (73ft l0in)
Length: 16m (52ft 6in)
Height: 4.35 m / 14 ft 3 in
Wing area: 69.6 sq.m / 749.17 sq ft

Ki-21-Ia (Army Type 97 Model 1A)
Engines: 2 x 850 hp Nakajima Ha 5 Kais.
Propellers: variable ¬pitch
Span: 22.5m (73ft l0in)
Length: 16m (52ft 6in)
Height: 4.35 m / 14 ft 3 in
Wing area: 69.6 sq.m / 749.17 sq ft
Gross weight: 7916 kg (17452 lb)
Empty weight: 4691 kg / 10342 lb
Maximum speed: 432 km/h (268 mph) at 4000 m (13,125 ft)
Ceiling: 8600 m / 28200 ft
Range w/max.fuel: 2700 km / 1678 miles
Range w/max.payload: 1500 km / 932 miles
Crew: 5
Armament: 3 x 7.7mm (0.303 in) Type 89 machine-guns
Bombload: 750 kg (1653 lb) – 1000 kg (2205 lb)

Ki-21-Ib
Engines: 2 x 634kW Nakajima Ha-5 KAI radials.
Propellers: variable ¬pitch
Span: 22.5m (73ft l0in)
Length: 16m (52ft 6in)
Height: 4.35 m / 14 ft 3 in
Wing area: 69.6 sq.m / 749.17 sq ft
Maximum range: 2700 km (1680 miles)
Armament: 4 x 7.7mm (0.303 in) Type 89 machine-guns
Bombload: 750 kg (1653 lb) – 1000 kg (2205 lb)

Ki-21-Ic
Engines: 2 x 634kW Nakajima Ha-5 KAI radials.
Propellers: variable ¬pitch
Span: 22.5m (73ft l0in)
Length: 16m (52ft 6in)
Height: 4.35 m / 14 ft 3 in
Wing area: 69.6 sq.m / 749.17 sq ft
Maximum range: 2700 km (1680 miles)
Armament: 4 x 7.7mm (0.303 in) Type 89 machine-guns
Bombload: 750 kg (1653 lb) – 1000 kg (2205 lb)

Ki 21-II
Engines: 2 x 1450 hp / 1119kW Mitsubishi Ha 101 14 cylinder radials
Props: constant speed three bladed metal
Span: 22.5m (73ft l0in)
Length: 16m (52ft 6in)
Height: 4.35 m / 14 ft 3 in
Wing area: 69.6 sq.m / 749.17 sq ft
Maximum range: 2700 km (1680 miles)
Armament: 4 x 7.7mm (0.303 in) Type 89 machine-guns
Bombload: 750 kg (1653 lb) – 1000 kg (2205 lb)

Ki-21-IIa (Army Type 97 Heavy Bomber Model 2A),
Engines: 2 x 1450 hp / 1119kW Mitsubishi Ha 101 14 cylinder radials
Props: constant speed three bladed metal
Span: 22.5m (73ft l0in)
Length: 16m (52ft 6in)
Height: 4.35 m / 14 ft 3 in
Wing area: 69.6 sq.m / 749.17 sq ft
Maximum range: 2700 km (1680 miles)
Armament: 4 x 7.7mm (0.303 in) Type 89 machine-guns
Bombload: 750 kg (1653 lb) – 1000 kg (2205 lb)

Ki-21-IIb
Engines: 2 x 1450 hp / 1119kW Mitsubishi Ha 101 14 cylinder radials
Props: constant speed three bladed metal
Span: 22.5m (73ft l0in)
Length: 16m (52ft 6in)
Height: 4.35 m / 14 ft 3 in
Wing area: 69.6 sq.m / 749.17 sq ft
Empty weight: 13,382 lb
Gross weight: 9710 kg (21407 lb)
Maximum speed: 486 km/h (302 mph) at 4720 m (15,485 ft).
Maximum range: 2700 km (1680 miles)
Armament: 4 x 7.7mm (0.303 in) Type 89 machine-guns, 1 x 12.7mm Type 1 heavy machine gun.
Bombload: 750 kg (1653 lb) – 1000 kg (2205 lb)
Crew: 7

MC 21

Kiro Honjo led the design team to a 12-Shi requirement of the Imperial Navy specifying the range, and twin engines. The result was very light and devoid of any protection for crew, engines, fuel tanks or systems.

The first flight by the first of two prototypes was made on October 23, 1939, to a 1937 requirement for a long-range bomber. During trials recorded an extraordinary performance of a 444km/h top speed and 5,555km range, albeit without bombload.

Mitsubishi G4M Article

Almost immediately afterwards its development was channelled in an entirely different direction. The company was instructed by the navy to adapt it instead for bomber escort duties. This meant sacrificing some 25% of the fuel load to offset the added weight of extra guns and ammunition, and increasing the crew to a total of ten men, compared with seven in the bomber version. No fewer than 30 examples of the G6M1, as this version was known, had been completed and put through service acceptance trials in 1940 before the JNAF was forced to admit that performance was just not good enough for the escort job. The aircraft themselves were later adapted to serve as G6M1 K trainers and, later still, as G6M1 1 2 paratroop transports.

Continuing G4M development, however, Mitsubishi managed to get another 14 of these bombers completed by the end of March 1941, and in April this version was accepted for service as the G4M1 Model 11. They were soon in successful operation against targets in China, and by the time of the Pearl Harbor attack at the end of that year, 120 or so were in service. The Model 11 was armed with four single 7.7 mm (0.303 in) Type 92 machine guns in nose, waist and dorsal positions, and a 20 mm (0.79 in) Type 99 cannon in the tail, and could carry the specified 800 kg (1764 1b) weapon load. Powerplant was a pair of 1530 hp Mitsubishi Kasei 11 14 cylinder two row radial engines. The Model 11 scored a number of early successes, but when losses began to mount, Mitsubishi produced the improved G4M1 Model 12, with Kasei 15 engines.

The first production G4M1s (Navy Type 1 Attack Bomber Model 11) were initially deployed against China in mid-1941 but on the eve of the attack on Malaya the bombers moved to Indo-China and within a week had successfully attacked the Prince of Wales and Repulse. When Allied fighter opposition eventually increased to effective proportions, the G4M1 was seen to be very vulnerable, possessing little armour protection for crew and fuel tanks, and it was in a pair of G4M1s that Admiral Yamamoto and his staff were travelling when shot down by P-38s over Bougainville on 18 April 1943. Little improvement had been secured in the Navy Type 1 Attack Bomber Model 22 with revised powerplant.

Its inherent weakness, which earned it the totally unwelcome sobriquet of ‘the one shot lighter’, was not altogether surprising when it is considered that the September 1937 JNAF requirement to which it was designed demanded a twin engined aircraft with a range of nearly 4830 km (3000 miles) and, over 3706 km (2300 miles), an 800 kg (1764 1b) weapon load. This ‘range at all costs’ philosophy could only be met by cramming every available bit of wing space with 4900 litres (1078 Imp gal) of fuel, and only then by omitting any kind of armour protection for either the crew or the fuel tanks. Everything else had to go into the fuselage and, by the standards of the day, was necessarily large and bulky.

After the Solomon Islands campaign in August 1942, when losses were particularly heavy because of the bomber’s extreme inflammability, there quickly followed the prototype G4M2.

This had more powerful engines, 1343kW / 1800 hp Kasei 21s with water¬methanol injection and several aerodynamic improvements. Armament was increased by adding two more 7.7 mm (0.303¬in) guns in the nose and replacing the dorsal 7.7 mm by a turret mounted 20 mm (0.79 in) cannon; internal bombload went up to 1000 kg (2204 lb). Fuel capacity was increased, to 6490 litres (1427 Imp gal), but still the tanks remained virtually unprotected. This version (Navy Type 1 Attack Bomber Model 22A and Model 22B) remained in production until the end of the war in steadily improved Navy Type 1 Attack Bomber Model 24 variants. The G4M2 was built in such numbers as to become the second most important production model, a total of 1154 being completed compared with 1200 G4M1s. These included the G4M2 Model 22A (Type 99 cannon in the waist positions) and 22B (all four Type 99 cannon of a later type); the G4M2a Model 24 (with 1850 hp Kasei 25 engines and bulged bomb bay doors); the G4M2b Model 25 testbed (1795 hp Kasei 27s); two G4M2c Model 26s (testbeds for turbocharged Kasei 25s); the G4M2d Model 27 testbed (1825 hp Kasei 25b engines); and the G4M2e Model 24.1 a version with one 7.7 mm (0.303 in) and four 20 mm (0.79 in) guns.

In the face of continuing heavy losses, in 1944 Mitsubishi built 60 examples of the G4M3, with armour protection for the crew and a much ¬redesigned wing containing a reduced fuel load of 4490 litres (988 Imp gal), stored in fully protected tanks, produced in small numbers as the Navy Type 1 Attack Bomber Model 34. Final versions were two G4M3 Model 36 prototypes, which made test flights with exhaust driven engine turbo¬chargers.

The ‘Betty’ did per¬form as the carrier aircraft for the air launched Yokosuka MXY¬7 Ohka piloted flying bomb, developed in the latter half of 1944. The Ohka should have entered service at the end of that year, but the first consignment of 50 was lost in November, when the carrier Shinano was sunk on its way to the Philippines. The first combat encounter was thus deferred until March 21, 1945, when a force of 16 Ohka¬-carrying G4M2e Model 24J bombers (the version chosen for this role), with an escort of 30 Zero fighters, was despatched against a US task force some 480 km (298 miles) off Kyushu. On the way, however, the force was met by more than 50 US Hellcat fighters, and lost every Ohka carrying bomber and half of the escorting Zeros. In order to attach the Ohka beneath the belly of the G4M2e the latter’s bomb bay doors were removed. Launch was usually made from an altitude of around 8200 m (26900 ft), at an airspeed of about 319 km/h (198 mph), the Ohka then gliding towards its target, cutting in the rocket motor only for the last few miles of flight and the terminal dive.

Many G4M1s were converted into trainers, maritime reconnaissance aircraft or 20 passenger troop transports towards the end of the Second World War. The G4M can also claim to have served from beginning to end of the Pacific war, for a pair of white painted G4M1s were used to trans¬port the Japanese delegation to Ie Shima on August 19, 1945, to sign the instrument of surrender.

Of the total production of 2446 G4M series aircraft, production amounted to 1,200 G4M1s, 1,154 G4M2s and 60 G4M3s.

Gallery

G4M1
Engine: 2 x Mitsubishi Kasei (Mars) II 14 cyl, 1530 hp
Crew: 7 Armament: 4 x 7.7mm mg, 1 x 20 mm cannon
Max speed: 266 mph at 13,870 ft
Cruise speed: 196 mph at 9840 ft
Service ceiling: 30,000 ft
Range: 1900 mile
Bomb load: 2200 lb or 1 x 1760 lb torpedo

G4M2
Crew: 7-10
Engines: 2 x Mitsubishi MK4P “Kasei-21”, 1350kW
Max take-off weight: 12500 kg / 27558 lb
Empty weight: 8160 kg / 17990 lb
Wingspan: 24.9 m / 81 ft 8 in
Length: 19.62 m / 64 ft 4 in
Height: 6 m / 19 ft 8 in
Wing area: 78.13 sq.m / 840.98 sq ft
Max. speed: 430 km/h / 267 mph
Cruise speed: 310 km/h / 193 mph
Ceiling: 8950 m / 29350 ft
Range w/max.fuel: 6000 km / 3728 miles
Armament: 2 x 20mm cannons, 4 x 7.7mm machine-guns, 2200kg of bombs

G4M2a Model 24
Engine: 2 x Mitsubishi Kasei 25, 1850 hp
Span: 24.89 m (81 ft 8 in)
Length: 19.63 m (64 ft 5 in)
Height: 13 ft 5.75 in
Empty weight: 18,449 lb
Gross weight: 15,000 kg (33 069 lb)
Maximum speed: 436 km/h (271 mph) at 15,090 ft
Service ceiling: 29,350 ft
Normal range: 2262 mi
Armament: 4 x 20mm Type 99 cannon, 1 x 7.7mm Type 97 mg
Bombload: 2200 lb or 1 x 1764 lb torpedo
Crew: 7