The Mitsubishi G4M was a twin-engine, land-based medium bomber formerly manufactured by the Mitsubishi Aircraft Company, a part of Mitsubishi Heavy Industries, and operated by the Imperial Japanese Navy from 1940 to 1945.
Its official designation is Mitsubishi Navy Type 1 attack bomber and was commonly referred to by Japanese Navy pilots as Hamaki due to the cylindrical shape of its fuselage.
Designed to a strict specification to succeed the Mitsubishi G3M already in service, the G4M boasted very good performance and excellent range and was considered the best land-based naval bomber at the time.
This was achieved by its structural lightness and an almost total lack of protection for the crew, with no armour plating or self-sealing fuel tanks.
The G4M was officially adopted on 2 April 1941 but the aforementioned problems would prove to be a severe drawback, often suffering heavy losses; Allied fighter pilots nicknamed the G4M “The Flying Lighter” as it was extremely prone to ignition after a few hits.
It was not until later variants of the G4M2 and G4M3 that self-sealing fuel tanks, armour protection for the crew and better defensive armament was installed.
Nevertheless, the G4M would become the Navy’s primary land-based bomber.
It is the most widely produced and most famous bomber operated by the Japanese during World War II, and it served in nearly all battles during the Pacific War.
The aircraft is also known for being the mothership that carried the Yokosuka MXY-7 Ohka, a purpose-built anti-ship suicide weapon during the final years of the war.
Of the 2,435 G4Ms produced, no intact aircraft have survived.
The G4Ms predecessor the Mitsubishi G3M went into service in 1937 in China.
Only two months later the Japanese Navy issued specifications to Mitsubishi.
The specifications, unprecedented at the time, called for a twin-engine, land-based, attack bomber with a top speed of 398 kilometres per hour (247 mph), altitude of 3,000 metres (9,800 ft), and a range of 4,722 kilometres (2,934 mi) unloaded (without bombs and torpedoes), and a range of 3,700 kilometres (2,300 mi) when carrying an 800 kilograms (1,800 lb) torpedo or the same weight in bombs.
The G4M was designed for a long range and high speed at the time of its introduction.
In order to meet the Navy’s specifications a Mitsubishi team led by Kiro Honjo did not incorporate self-sealing fuel tanks and armour plating to save weight and extend range.
This consequently made both the G4M and the Zero, in which Mitsubishi used the same design features, vulnerable to machine gun and cannon fire.
The pilots of the Imperial Japanese Navy called the G4M the “hamaki” (“cigar”), although this was due to its shape.
Due to deficiencies of the G3M in warding off concentrated fighter attacks Honjo incorporated 7.7 mm (0.30 in) guns in the nose, on top and both sides of the fuselage and in the tail a 20 mm (0.79 in) cannon was added.
When used for medium to high-altitude bombing against stationary land targets like supply depots, seaports or airfields, it was much harder to intercept.
Using its long range and high speed, the G4M could appear from any direction, and then it could be gone before any fighters intercepted them.
The 20 mm cannon in its tail turret was much heavier armament than was commonly carried by bombers of either side, making aerial attacks from the rear quite dangerous for the Allied fighter aircraft.
Sometimes, if they did not catch fire after being hit in the wings by flak from the ground or by machine gun bullets from enemy fighters, G4Ms also proved to be able to remain airborne despite being badly damaged.
For example, after the attack of the 751 Kōkūtai (air group) on the USS Chicago during the Battle of Rennell Island, three out of four surviving aircraft (of the original eleven) returned despite flying with only one engine.
As the war continued improved bomber designs failed to materialize and Mitsubishi began creating additional versions to fulfill various new missions as well as eliminate the weakness in the design including various engine and weapon variants.
The G4M2 redesign failed to rectify the G4M’s vulnerability to weapons fire.
(Mitsubishi Navy Type 1 attack bomber) / (Mitsubishi Navy Experimental 12-Shi land attacker).
Two prototypes built.
G4M1 Model 11
(Mitsubishi Navy Type 1 Attack Bomber Model 11).
The first bomber model of series, with 1,140 kW (1,530 hp) Mitsubishi MK4A “Kasei” Model 11 engines driving three-blade propellers.
The following modifications were made during production
The first aircraft (241st production example) fitted with Mitsubishi MK4E “Kasei” Model 15 engines with larger superchargers for better high-altitude performance, became standard in August 1942 from 406th aircraft onwards.
These MK4E-engined aircraft have often (erroneously) been referred as the “G4M1 Model 12”.
Propeller spinners introduced
From 663rd machine onwards, 30 mm (1.181 in) rubber ply sheets installed beneath the wing outer surfaces to protect the underside of the fuel tanks (speed reduced by 9 km/h (4.9 kn; 5.6 mph) and range by 315 km (170 nmi; 196 mi), 5 mm (0.197 in) armour plates added into tail gunner’s compartment.
Outer half of the tail cone cut away in order to improve tail gunner’s field of fire.
A completely redesigned tail cone, with reduced framing and wide V-shaped cut out; this form of tail cone was also used in all G4M2 models.
Individual exhaust stacks from 954th airframe onwards
Production of the G4M1 ended in January 1944.
The first of the four G4M2 prototypes flew in December 1942 (
Mitsubishi Navy Type 1 Attack Bomber Model 22).
It differed from the preceding model in having Mitsubishi MK4P “Kasei” Model 21 engines with VDM electric four-blade propellers capable of full feathering function, redesigned main wings with LB type laminar flow airfoil and widened tail horizontal stabilizer wing area, which improved service ceiling to 8,950 m (29,360 ft) and maximum speed to 437 km/h (236 kn; 272 mph).
Main wing fuel tanks were enlarged to 6,490 l (1,710 US gal; 1,430 imp gal) which increased the range to 6,000 km (3,200 nmi; 3,700 mi) (overloaded, one way).
An electrically powered dorsal turret featuring a 20 mm (0.787 in) Type 99 cannon was introduced in place of G4M1’s dorsal position with a 7.7 mm (0.303 in) Type 92 machine gun, total guns armed were two 20 mm (0.787 in) Type 99 cannons (one tail turret, one top turret), and four 7.7 mm (0.303 in) Type 92 machine guns (one nose, two waists, and one cockpit side).
External differences also included increased nose glazing, flush side gun positions instead of blisters, and rounded tips of wings and tail surfaces.
These major improvements also made it possible for the G4M2 to carry more powerful bombs; one 1,055 kg (2,326 lb) Navy Type 91 Kai-7 aerial torpedo or one 800 kg (1,800 lb) bomb or two 500 kg (1,100 lb) bombs or one 800 kg (1,800 lb) Type 3 No. 31 bomb (ray-detective type bomb) and twelve 60 kg (130 lb) bombs.
The G4M2 entered service in mid-1943.
G4M2 Model 22
(Mitsubishi Navy Type 1 Attack Bomber Model 22)
The base model, the first production example completed in July 1943.
Introduced bulged bomb bay doors from 65th aircraft onwards, and an optically flat panel in the nose cone from the 105th aircraft onwards.
G4M2 Model 22Ko
(Mitsubishi Navy Type 1 Attack Bomber Model 22 Ko)
Very similar to previous model.
Carried Type 3 Ku Mark 6 search radar and was armed with 20 mm (0.787 in) Type 99 Model 1 cannons replacing the 7.7 mm (0.303 in) Type 92 machine guns in the lateral positions.
G4M2 Model 22 Otsu
(Mitsubishi Navy Type 1 Attack Bomber Model 22 Otsu)
Dorsal turret cannon changed to longer-barrelled 20 mm (0.787 in) Type 99 Model 2 cannon.