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.
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.
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
