The G4M is one of Japan’s most famous Second World War aircraft, sitting up there alongside the A6M Zero as an icon of the Pacific War.
Developed by the Mitsubishi Aircraft Company for the Imperial Japanese Navy, the G4M served as a long-range, land-based bomber, playing a pivotal role in Japan’s aerial operations across the Pacific Theatre. It was designed for speed and massive range, but to achieve this, sacrifices had to be made.
Unfortunately for crews, this was the omission of armor and protected fuel tanks, resulting in a very delicate machine that had a tendency to ignite when shot at.
Contents
Background
In the 1930s, as Japan expanded its territorial ambitions in Asia and the Pacific, the need for a capable long-range bomber became evident. The Imperial Japanese Navy sought an aircraft that could support its operations over the vast distances of the Pacific Ocean, providing the reach and payload necessary to project Japanese power far from the home islands.
G3M “Nell”
Japanese manufacturer Mitsubishi initially responded to this with the G3M medium bomber, which entered service in 1937. The G3M’s design aimed to maximize range and speed, prioritizing these characteristics over heavy armament and armor protection – a design philosophy that would become a hallmark of Japanese military aircraft.
The aircraft featured a low-wing monoplane design, with two powerful Mitsubishi Kinsei radial engines enabling it to achieve the long ranges required for operations across the vast expanses of the Pacific. Its slender, streamlined fuselage was designed for efficiency, and it was equipped with a retractable undercarriage.
The G3M could carry a moderate bomb load and was armed with defensive machine guns, but its light defensive armament and lack of armor made it vulnerable to enemy fighters and anti-aircraft fire.
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But shortly after the G3M entered service, the Imperial Japanese Navy released a new set of requirements for an aircraft with even greater range, bomb load and speed. They were not easy; the new aircraft needed to be able to cruise at an altitude of 3,000 m (9,845 ft) while flying at a top speed of 247 mph (398 kph) for a distance of 2,933 miles (4,722 km) without a torpedo or equivalent weight in bombs.
When carrying an 800 kg (1,768 lb) torpedo or load of bombs, it still needed a range of 2,300 miles (3,700 km). Mitsubishi Heavy Industries got to work on an aircraft that could satisfy these difficult demands. Once again, development occurred under the leadership of Honjo. The solution was the G4M, designated “Betty” by the Allies.
As Light as Possible
The design philosophy behind the G4M was shaped by the strategic imperatives of range and payload. To meet these requirements, Honjo and his team made significant trade-offs, prioritizing the aforementioned attributes over capacity over defensive capabilities. This meant discarding armor plating and self-sealing fuel tanks.
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This may seem absurd, but at the time this approach was rooted in the belief that speed and operational range would enable the G4M to outrun and outmaneuver the enemy, reducing the need for heavy armament or armor in the first place.
G4M Design
The G4M was a twin-engine, land-based medium bomber optimised for long range and high speed.
It had a low-wing monoplane configuration, which was selected for its aerodynamic efficiency. The wing itself was relatively thin and had a high aspect ratio, reducing drag and enhancing the aircraft’s range and fuel efficiency.
These wings were equipped with large, high-lift devices, including flaps and leading-edge slats, to improve takeoff, landing performance, and low-speed handling, crucial for operations from shorter runways that were common on forward island bases.
The fuselage of the G4M was streamlined and sleek, designed to minimize frontal area and thus reduce aerodynamic drag. This slim design was also a consequence of the desire to maximize fuel load within the aircraft’s structure, a factor critical to achieving the long range required by the Japanese Navy.
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It was famously referred to as a “cigar” thanks to its distinct fuselage shape. Inside were a crew of seven: pilot, co-pilot, navigator, dorsal turret gunner, two waist gunners and a tail gunner.
The aircraft’s structure made extensive use of light alloys, which helped keep the overall weight down, although this choice later contributed to the aircraft’s vulnerability to fire and damage in combat due to the lack of self-sealing fuel tanks and minimal armor protection for the crew and vital components.
It was respectably large aircraft, with a fuselage length of 20 meters (65 ft 6 in) and a wingspan of 25 meters (81 ft 8 in). This roughly places it between the B-25 and B-17 in terms of size. Its maximum take off weight was close to 30,000 kg.
Powerplant
The G4M was powered by two Mitsubishi MK4A Kasei 11 14-cylinder radial engines, which provided 1,500 hp each. These engines were capable of propelling the G4M to speeds of up to 435 kph (270 mph), making it one of the faster bombers of the early war years.
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The engine nacelles were mounted on the leading edge of the wings and contributed to the aircraft’s sleek profile. The choice of radial engines was influenced by their light wight, reliability and the relative ease of maintenance, crucial factors for operations in the remote and often harsh environments of the Pacific theater.
They can also take quite a beating from enemy fire as they do not have a vulnerable liquid cooling system.
Armament and Defense
The G4M featured an internal bomb bay in the fuselage. This could be used to carry bombs or torpedos, with a typical load usually consisting of 800 kg (1,760 lbs) of bombs, or a single Type 91 torpedo for anti-shipping missions.
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Defensively, the G4M was initially equipped with a relatively light armament of machine guns located in the nose, dorsal, waist and tail positions. Later versions saw improvements in armament, including the addition of a 20 mm cannon in the tail turret to better fend off attacking fighters.
However, the aircraft’s defensive capabilities were inherently limited by the design’s prioritization of range and speed over survivability.
A Delicate Design
The G4M’s design, while innovative, entailed significant, famous vulnerabilities. The lack of armor protection for the crew and critical components, combined with the absence of self-sealing fuel tanks, made the aircraft highly susceptible to enemy fire.
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Reports from the war noted that G4Ms could be easily ignited by a few well-placed bullets or cannon shells, earning it the grim nickname “flying lighter” among Allied pilots. On the other hand, if the aircraft was not set ablaze during an attack, it had quite a high chance of staying in the air as the airframe itself was impressively rugged.
Service Life
The G4M first flew on October 23, 1939 and proved to be a fast, well-designed machine. Despite this, work on the bomber was suddenly stopped in favour of another variant with more weaponry on board. After this aircraft failed to materialise, focus returned to the G4M and it entered service in 1941.
The new bomber quickly became the mainstay of the IJNAS’s long-range bombing capability. In its initial operations the G4M performed very well, fighting in the most ideal conditions for the bomber. It meshed perfectly into the strategic doctrine of Japan at the time as designed, which emphasized rapid expansion and the need to project air power over vast distances.
The G4M’s long range and substantial payload capacity made it an ideal platform for pre-emptive strikes and reconnaissance missions deep into enemy territory.
Early Successes
The G4M’s service history began with notable successes that leveraged its range and speed. One of its earliest and most infamous uses was in the attack on Clark Air Base in the Philippines, immediately following the Pearl Harbor attack in December 1941.
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This operation showcased the G4M’s capability to strike far from its bases, a tactic that became a hallmark of Japanese operations in the early stages of the war.
Another significant early operation involving the G4M was the sinking of the British battleship HMS Prince of Wales and the battlecruiser HMS Repulse off the coast of Malaya in December 1941. The G4Ms, armed with torpedoes, demonstrated the potential of land-based bombers to alter the course of naval engagements.
As the war progressed, the G4M was adapted to fulfill a variety of roles beyond conventional bombing. Its long range made it suitable for maritime reconnaissance, troop transport, and even as a launch platform for the Yokosuka MXY-7 Ohka, a manned kamikaze attack aircraft.
Vulnerabilities Exposed
Despite its early successes, the G4M’s operational history was also marked by increasing losses due to its design vulnerabilities. The lack of self-sealing fuel tanks and insufficient armor protection made it highly susceptible to enemy fire.
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Allied pilots soon recognized the G4M’s propensity to catch fire, earning it nicknames such as “flying lighter” and “one-shot lighter.” These vulnerabilities became more pronounced as the Allies improved their air defenses and fighter tactics.
The Battle of Midway in June 1942 was a turning point in the Pacific War and for the G4M’s service history. The loss of four Japanese aircraft carriers, partly due to the effective counterattacks by U.S. aircraft, highlighted the limitations of the Japanese strategy and the G4M’s vulnerabilities. Subsequent operations saw the G4M suffering heavy losses, particularly in engagements with more advanced American fighters such as the F6F Hellcat and the F4U Corsair.
Famous and Final Actions
The aircraft was famously at the center of the death of Japanese Admiral Isoroku Yamamoto, one of the masterminds behind the 1941 Attack on Pearl Harbor. He was intercepted and shot down by American P-38 Lightnings while being flown in a G4M on April 18, 1943.
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As the war turned against Japan, the G4M continued to be used in increasingly desperate operations, including kamikaze missions. The introduction of the G4M2 model, which featured improved engines and slightly better defensive armament, did little to change the aircraft’s fundamental vulnerabilities.
In the war’s final stages, the G4M was often used for night bombing raids and special missions, attempting to avoid the overwhelming Allied air superiority during daylight hours.
The final moment for the G4M was arguably its most important. It was the aircraft of choice used to transport the Japanese delegation to Manilla for the signing of Japan’s surrender to the Allies. These aircraft, named Bataan 1 and Bataan 2, were painted while and marked with green crosses. They were destroyed after the flight.
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In total, just over 2,400 G4M bombers were built.
