Mitsubishi G4M


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. The Allied reporting name was "Betty".
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 armor 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, armor 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 2400 G4Ms produced, no intact aircraft have survived.

Design and development

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, altitude of, and a range of unloaded, and a range of when carrying an 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 lead by Kiro Honjo did not incorporate self-sealing fuel tanks and armor plating to save weight and extend range. This consequently made both the G4M and the Zero, which Mitsubishi used the same design features, vulnerable to machine gun and cannon fire. Consequently, Allied fighter pilots to give it derisive nicknames such as "the flying lighter" "the one-shot lighter", "the flying Zippo" and "the flying cigar" because of their tendency to ignite from damage to the wing fuel tanks after being hit by gunfire. The pilots of the Imperial Japanese Navy called the G4M the "hamaki", however this was due to its shape. Due to deficiencies of the G3M in warding off concentrated fighter attacks Honjo incorporated guns in the nose, on top and both sides of the fuselage and in the tail a cannon was added.
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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, assuming 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 on the USS Chicago during the Battle of Rennell Island, three out of four surviving aircraft 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 news missions as well as eliminate the weakness in the design including various engine and weapon variants. The G4M2 redesign failed to shore up the G4Ms vulnerability to weapons fire.

First flight

The first G4M prototype left Mitsubishi's Nagoya plant in September 1939 disassembled and loaded in five ox-drawn farm carts to Kagamigahara airfield to the north. On October 23, 1939, test pilot Katsuzo Shima flew the G4M prototype. Despite successful tests the Navy shelved the bomber for the more heavily armed G6M1 variant in hopes it could be used as heavy escort fighter for other bombers. Failing these expectations the G4M1 was ordered into production.

Production

The first production G4M was completed in April 1941 and was not discontinued until the end of the war.
The G4M was similar in performance and missions to other contemporary twin-engine bombers such as the German Heinkel He 111 and the American North American B-25 Mitchell. These were all commonly used in anti-ship roles. The G4M Model 11 was prominent in attacks on Allied shipping from 1941 to early 1944, but after that it became increasingly easy prey for Allied fighters.
The G4M was first used in combat on 13 September 1940 in Mainland China, when 27 "Bettys" and Mitsubishi C5Ms of 1st Rengo Kōkūtai departed from Taipei, Omura, and Jeju City to attack Hankow. The bombers and the reconnaissance aircraft were escorted by 13 A6M Zeros of 12th Kōkūtai led by the IJN lieutenant, Saburo Shindo. A similar operation occurred in May 1941. In December 1941, 107 G4Ms based on Formosa of 1st Kōkūtai and Kanoya Kōkūtai belonging to the 21st Koku Sentai crossed the Luzon Strait en route to bombing the Philippines; this was the beginning of Japanese invasions in the Southwest Pacific Theater.
In its first year of combat the G4M was a success, attacking U.S. Army air base Clark Field, Philippines on December 8, 1941 and taking part in the operation to sink HMS Prince of Wales and HMS Repulse two days later. Against weak fighter opposition the G4M attacked targets ranging as far as the Aleutians to Australia using its long range, the drawbacks of no self-sealing fuel tanks and armor not presenting themselves as problems at this point.
The G4M's most notable use as a torpedo bomber was in the sinking of Prince of Wales and Repulse off the eastern coast of Malaya on 10 December 1941. The G4Ms attacked along with older Mitsubishi G3M "Nell" bombers, which made high-level bombing runs. Prince of Wales and Repulse were the first two capital ships to be sunk exclusively by air attacks during a war, while in open waters. The bomber crews were from the Kanoya Air Group, Genzan Air Group, and the Mihoro Air Group, trained in torpedo attacks at an altitude of less than, and in long-range over-ocean navigation, so they could attack naval targets moving quickly at sea. Nine G4Ms participated in the long-range bombing raid of Katherine, Northern Territory, on 22 March 1942. G4Ms later made many attacks against Allied ships and also land targets during the six-month-long Guadalcanal Campaign in late 1942.
On 8 August 1942, during the second day of the U.S. Marine landings on Guadalcanal, 23 IJNAF torpedo-carrying G4M1s attacked American ships at Lunga Point. 18 of the G4M1s were shot down, by very heavy anti-aircraft fire and carrier-based F4F fighters. In all 18 Japanese crews – approximately 120 aviators – were lost at the beginning of August 1942. More than 100 G4M1s and their pilots and crews were lost during the many battles over and near Guadalcanal. In the two days of the Battle of Rennell Island, 29 and 30 January 1943, 10 out of 43 G4M1s were shot down during night torpedo attacks, all by U.S. Navy anti-aircraft fire. About 70 Japanese aviators, including Lieutenant Commander Higai, were killed during that battle.
Probably the best-known incident involving a G4M during the war was the attack resulting in the death of Admiral Yamamoto. On 18 April 1943, sixteen P-38 Lightnings of the 339th Fighter Squadron of the 347th Fighter Group, Thirteenth Air Force, shot down a G4M1 of the 705th Kōkūtai with the tailcode "T1-323", carrying Admiral Yamamoto.
The G4M Model 11 was replaced by the Models 22, 22a/b, 24a/b, 25, 26, and 27 from June 1943 onward, giving service in New Guinea, the Solomons, and the South Pacific area, in defense of the Marianas and finally in Okinawa. Other G4Ms received field modifications, resulting in the Model 24j. This model carried the Yokosuka MXY7 Ohka Model 11 suicide flying bomb, beginning on 21 March 1945, with disastrous results due to heavy Allied fighter opposition.
After the loss of Okinawa, G4Ms constituted the main weapon of the land-based Japanese naval bomber force. It consisted of 20 Kōkūtai at the end of the war. This included the testing air group, which was equipped in 1944–45 with the latest version G4M3 Models 34 and 36, though these arrived too late to affect the course of the war.
From November 1944 to January 1945, G4Ms were one of the main types of aircraft used in the Japanese air attacks on the Mariana Islands, and plans to use converted G4Ms to land commandos on the islands were developed in mid-1945 and cancelled only at the end of the war.
As part of the negotiations for the surrender of Japan, two demilitarized G4Ms, given the call-signs Bataan 1 and Bataan 2, flew to Ie Shima, carrying the first surrender delegations on the first leg of their flight to Manila. The G4Ms were painted white with green crosses, and were escorted by American P-38 fighters.
The G4M's intended successor was the Yokosuka P1Y Ginga, although because of production problems, the changeover was only begun by the time the war ended.

Variants

G4M1

;G4M1 prototypes: /. Two prototypes built.
;G4M1 Model 11:. The first bomber model of series, with Mitsubishi MK4A "Kasei" Model 11 engines driving three-blade propellers. The following modifications were made during production:
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 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 and maximum speed to. Main wing fuel tanks were enlarged to which increased the range to . An electrically powered dorsal turret featuring a Type 99 cannon was introduced in place of G4M1's dorsal position with a Type 92 machine gun, total guns armed were two Type 99 cannons, and four Type 92 machine guns. 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 Navy Type 91 Kai-7 aerial torpedo or one bomb or two bombs or one Type 3 No. 31 bomb and twelve bombs. The G4M2 entered service in mid-1943.
"Baka"
;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 Type 99 Model 1 cannon s replacing the Type 92 machine guns in the lateral positions.
;G4M2 Model 22 Otsu: dorsal turret cannon changed to longer-barreled Type 99 Model 2 cannon.
;G4M2a Model 24: modified Model 22, Mitsubishi MK4T Kasei 25 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: armament similar to Model 22 Ko.
;G4M2a Model 24 Otsu: armament similar to Model 22 Otsu.
;G4M2a Model 24 Hei: modified 24 Otsu, with one 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 Mitsubishi MK4T-B Kasei 25 Otsu engines. Only experimental.
;G4M2c Model 26: two G4M2as modified to Mitsubishi MK4T-B Ru Kasei 25b engines with turbochargers.
;G4M2d Model 27: one G4M2 modified to Mitsubishi MK4V Kasei 27 engines.
;G4M2e Model 24 Tei: special version for the transport of the ramming attack bomb plane Kugisho/Yokosuka MXY-7 Ohka Model 11, conversions of G4M2a Model 24 Otsu and 24 Hei. Had armour protection for the pilots and fuselage fuel tanks.
;MXY11 : 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 two Type 92 machine guns in nose cabin and in both side positions, and one Type 99 Model 1 cannon in dorsal turret and tail. Entered production in October 1944 in G4M3a Model 34 Ko form with Type 99 cannon in side positions instead of machine guns.
;G4M3a Model 34 Hei: similar modifications as in corresponding Model 24 variants.
;G4M3a Model 34 Otsu: similar modifications as in corresponding Model 24 variants.
;G4M3 Model 36: prototype. Two G4M2 Model 34 modified to Mitsubishi MK4-T Kasei 25b Ru engines.

G6M1

;G6M1: initial model of the series, armed with three Type 99 cannon and one Type 92 machine gun in the nose; 30 built.
;G6M1-K: trainers converted from G6M1s.
;G6M1-L2: G6M1s modified as transports.

Operators

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No flyable Mitsubishi G4Ms are left although several wrecks remain scattered in southeast Asia and on Pacific islands.
Several other locations display pieces of G4Ms including the restored fuselage of a G4M2 is on display at the Kawaguchiko Motor Museum in Yamanashi Prefecture, Japan.
Additionally the Smithsonian Institution retains the forward fuselage of a G4M3 Betty Model 34. Likely based in Oppama Air Field near Yokosuka, Japan there is no recorded tail number. The aircraft was part of 145 other Japanese aircraft for tests and evaluations by the U.S. Navy. After being flight tested as "Foreign Equipment Test number T2-2205" the airplane was dismembered by a cutting torch for unknown reasons.

Specifications (G4M1, Model 11)

Citations