Few would know it by its official designation, the Navy Type 1 Attack Bomber. The Allies called it the BETTY but to the men that flew the airplane, it was popularly, but unofficially, the 'Hamaki,' Japanese for cigar, in honour of the airplane's rotund, cigar-shaped fuselage. The Japanese built more of them than any other bomber during World War II. From the first day of war until after the surrender, BETTY bombers saw service throughout the Pacific and Indian Oceans. Like its stable mate, Mitsubishi's Zero Fighter the Hamaki soldiered on long after it became obsolete, even dangerous, to fly wherever Allied interceptors prowled.
In July 1937, the new Mitsubishi G3M bomber (Allied codename NELL) went into service in China. Only two months later, the Navy issued a specification to Mitsubishi for a NELL replacement. At that time, the requirements were unprecedented for a twin-engine, land-based attack bomber: flying at a top speed of 398 kph (247 mph) and an altitude of 3,000 m (9,845 ft), the new bomber had to fly a distance of 4,722 km (2,933 miles) without a torpedo or equivalent weight in bombs. When carrying an 800 kg (1,768 lb) torpedo or the same weight in bombs, the Navy needed the bomber to fly at least 3,700 km (2,300 mi).
To meet the requirements, a Mitsubishi design team led by Kiro Honjo crafted an airplane called the G4M with fuel tanks in the wings that were not resistant to explosion when punctured during combat. These tanks were much lighter in weight than explosion-proof (also called 'self-sealing') gas tanks. The decision not to incorporate the heavier, safer fuel tanks was necessary to meet the Navy's range requirements. Mitsubishi incorporated this same design feature in the Zero, for the same reasons and with the same results. Both aircraft had unprecedented range but they were also extremely vulnerable to the machine gun and cannon fire from Allied fighter aircraft. The BETTY was so prone to ignite that the Allies nicknamed it the 'flying lighter.'
The fuselage was streamlined but rotund to allow space for a bomb bay within the wing centre section and to allow the 7 to 9-man crew to move about. About half the crew were gunners who manned the defensive armament positions. Bomber crews flying the NELL were virtually incapable of defending themselves from concentrated fighter attacks, so Honjo paid special attention to this aspect of the G4M. He incorporated 7.7 mm (.30 cal.) guns in the nose, atop the mid-fuselage behind the cockpit, and on both sides of the fuselage behind the wing. In the tail, he introduced a 20 mm cannon. Although the G4M now had a more potent sting, Honjo again sacrificed crew protection to the Navy's demands for great range. He omitted armour plate.
The first G4M prototype left the factory in September 1939 and made the trek to Kagamigahara Airfield for Mitsubishi's Nagoya plant had no company airstrip. Kagamigahara was 48 km (30 miles) to the north. Japan's newest and most advanced bomber made the trip, disassembled and stacked on five ox-drawn farm carts, over unpaved roads! After arriving at the airfield, the first G4M was reassembled and flown by test pilot Katsuzo Shima on October 23, 1939. Initial results were impressive, but the Navy shelved the bomber for a time in favour of a variant to be called the G6M1. Navy leaders hoped that by increasing the number of defensive cannons, the G6M1 could become a heavy escort fighter for other bombers but this diversion failed to live up to expectations, and the Navy ordered the G4M1 into production. The U. S. Army Air Corps conducted a similar experiment using a modified Boeing B-17 bomber designated the B-40 but this idea too failed to survive operational testing and was soon abandoned. The first production G4M rolled off the line in April 1941. For the remainder of the war, the BETTY assembly line continued to run.
Operationally, BETTY crews achieved much in their first year of combat. They devastated Clark Field, Philippine Islands, on December 8, 1941, and participated in sinking the British battleships HMS "Prince of Wales" and HMS "Repulse" on December 10. They ranged across the length and breadth of the Pacific theatre, attacking targets from the Aleutians to Australia. Against limited fighter opposition, the lack of armour and self-sealing fuel tanks was no hindrance. The savings in airframe weight allowed the G4M to attack targets at unprecedented ranges. But as Allied fighter strength increased, the BETTY began to reveal its fatal vulnerabilities. Admiral Isoroku Yamamoto, architect of the Pearl Harbour attack, died on April 18, 1943, along with his entire staff when U. S. Army Air Corps P-38 Lightnings intercepted and destroyed the two BETTY bombers that carried them. Six escorting Zeros flew guard but in a matter of seconds, the Air Corps pilots shrugged off the escorting fighters and sent both BETTYs crashing down in flames.
As the war dragged, improved bombers failed to materialize so Mitsubishi fielded different versions of the G4M to fulfil new missions, and to eliminate the various weaknesses in the basic design. Front-line combat units operated many variants and sub-variants with different engines and armament packages. The G4M2 was a complete redesign but it did not overcome the airplane's vulnerability to Allied firepower. Mitsubishi tried again to reduce the bomber's tendency to burn. The firm changed the wing to a single-spar configuration and installed self-sealing fuel tanks with a capacity about one-third less than earlier versions. The capacity dropped because of the material inserted in the tank to block leaking fuel when gunfire perforated the tank. Armour plate was also added to all crew positions and the tail turret was redesigned. As a result of these modifications, the fuselage was shortened and the centre-of-gravity shifted forward. To re-balance the bomber, dihedral was added to the horizontal stabilizer. This version was called the G4M Model 34.
VERSIONS
An early-production Mitsubishi G4M1 Model 11 without the propeller spinners
G4M1
G4M1 Prototypes
Japanese Navy land Based Bomber Type 1. Two prototypes built.
G4M1 Model 11
Japanese Navy Land Attack Bomber Type 1. The first bomber model of series, with 1,140 kW (1,530 hp) Mitsubishi MK4A Kasei Model 11 engines driving three-blade propellers. Following modifications were made during the production:
March 1942: The first aircraft (241st production example) fitted with 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.
Summer 1942: Propeller spinners introduced.
March 1943: From 663rd machine onwards, 30 mm (1.18 in) rubber ply sheets installed beneath the wing outer surfaces to protect the undersides of the fuel tanks (speed reduced by 9 km/h/6 mph and range by 315 km/196 mi), 5 mm (.2 in) armour plates added into tail gunner's compartment.
Spring 1943: Outer half of the tail cone cut away in order to improve tail gunner's field of fire.
August 1943: 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.
September 1943: Individual exhaust stacks from 954th airframe onwards.
Production of the G4M1 ended in January 1944.
G4M2
The first of the four G4M2 prototypes flew in December 1942. It differed from the preceding model in having 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 (1,715 US gal) which increased the range to 6,100 km (3,790 mi/ 3,294 nmi overloaded, one way). An electrically powered dorsal turret featuring a 20 mm cannon was introduced in place of G4M1's dorsal position with a 7.7 mm machine gun, total guns armed were 2 × 20 mm Type 99 cannon (1 × tail turret, 1 × top turret), 4 × 7.7 mm Type 92 machine gun (1 × nose, 2 × waist, 1 × 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; 1 × 1,055 kg (2,326 lb) Type 91 Kai-7 (improved model 7) aerial torpedo or 1 × 800 kg (1,760 lb) bomb or 2 × 500 kg (1,100 lb) bombs or one Type 3 – 800 kg (1,760 lb) no.31 ray-detective type bomb + 12 × 60 kg (130 lb) bombs. This model, G4M2, was put into service in mid-1943.
G4M2 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
Very similar to previous model. Carried Type 3 Ku Mark 6 search radar and was armed with two 20 mm Type 99 Mark 1 cannons replacing the 7.7 mm machine guns in the lateral positions.
G4M2 Model 22 Otsu
Dorsal turret cannon changed to longer-barreled 20 mm Type 99 Mark 2.
G4M2a Model 24
Modified Model 22, MK4T Kasei 25 1,340 kW (1,800 hp) engines, with bulged bomb bay doors as standard for larger bomb capacity. Externally distinguishable from the Model 22 by a carburetor air intake on the top of the engine cowling.
G4M2a Model 24 Ko/Otsu
Armament similar to Model 22 Ko/Otsu respectively.
G4M2a Model 24 Hei
Modified 24 Otsu, with one 13.2 mm (.51 in) Type 2 machine gun mounted in tip of the nose cone, radar antenna relocated from that position to above the nose cone.
G4M2b Model 25
One G4M2a modified to MK4T-B Kasei 25 Otsu 1,360 kW (1,825 hp) engines. Only experimental.
G4M2c Model 26
Two G4M2a modified to MK4T-B Ru Kasei 25b 1,360 kW (1,825 hp) engines with turbochargers.
G4M2d Model 27
One G4M2 modified to MK4V Kasei 27 1,340 kW (1,800 hp) engines.
G4M2e Model 24 Tei
Special version for the transport of the ramming attack bomb plane Kugisho/Yokosuka MXY-7 Ohka ("Baka") Model 11, conversions of G4M2a Models 24 Otsu and 24 Hei. Had armour protection for the pilots and fuselage fuel tanks.
MXY11 Yokosuka Navy Type 1 Attack Bomber
Ground Decoy Non-flying replica of Mitsubishi G4M2 developed by Yokosuka
G4M3
G4M3 Model 34
Redesigned G4M2 with added self-sealing fuel tanks, improved armor protection and an entirely new tail gunner's compartment similar to that of late model B-26 Marauders. Wings were also redesigned and the horizontal tailplane was given dihedral. Armed with 2 × 7.7 mm Type 92 machine guns in nose cabin and in both side positions, and 1 × 20 mm Type 99 Model 1 cannon in dorsal turret and tail. Entered production in October 1944 in G4M3a Model 34 Ko form with 20 mm cannon in side positions instead of machine guns.
G4M3a Model 34 Otsu and Hei
Similar modifications as in corresponding Model 24 variants.
G4M3 Model 36
Prototype. Two G4M2 Model 34 modified to Mitsubishi MK4-T Kasei 25b Ru 1,360 kW (1,825 hp) engines.
G6M1
G6M1 Japanese Navy Long Range Heavy Fighter Type 1
Initial model of the series, armed with 20 mm Type 99 cannons between each side of fuselage and in tail, 1 × 7.7 mm machine gun in nose cabin and 1 × 30 mm cannon in front ventral position; 30 built.
G6M1-K Trainer, Japanese Navy Type 1
Converted G6M1s.
G6M1-L2 Transport Type 1, Japanese Navy
Modified as transports.
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