Mitsubishi G4M

Japan's naval strategy hinged on distance. To strike across the vast reaches of the Pacific, the Imperial Japanese Navy demanded a land-based bomber of extraordinary range — one able to range hundreds of kilometres out to sea, find an enemy fleet, and attack it with bombs or torpedoes. In 1937 the Navy issued just such a specification, and the task fell to Kiro Honjo and his design team at Mitsubishi, fresh from the earlier G3M.

The result, first flown in October 1939, was a sleek twin whose deep rounded fuselage earned it the crew name Hamaki — the Cigar. It carried a single aerial torpedo or up to a tonne of bombs on two Mitsubishi Kasei radials, handled well, and had respectable defensive armament for its day including a 20 mm cannon in the tail. Everything about it followed from the range requirement, including the decision that would define its reputation.

To reach the distance the Navy wanted, Honjo’s team filled the wings with enormous integral fuel tanks and left out both self-sealing liners and fire suppression, along with most crew armour. The wing was not a structure containing tanks; the wing was the tank. It bought the range, and it meant that a few incendiary rounds anywhere in the wing would set the aircraft alight.

Allied fighter pilots worked this out quickly and named it the “one-shot lighter”. Its own crews reached the same conclusion and called it the “Flying Lighter”. The problem was structural rather than a matter of fitting a part: protecting those tanks meant losing the volume the whole design existed to provide. Mitsubishi went at the problem three times — the G4M2, the G4M2a and finally the G4M3 — and only the last of them accepted the loss of range that a protected wing required.

On 10 December 1941 land-based naval bombers found and sank HMS Prince of Wales and HMS Repulse off Malaya — the first time capital ships under way at sea had been destroyed by air power alone. It is remembered as the Betty’s battle, and the strike composition corrects that. The force that flew from Saigon numbered 84 aircraft: 34 level bombers and 50 torpedo aircraft, and of the torpedo carriers only 25 were G4M. The rest, and every one of the level bombers, were the older G3M.

The G4M was therefore under a third of the force that made the most famous demonstration of land-based air power in the war, flying alongside the aircraft it had been designed to replace. What the day did prove was the specification: naval bombers had ranged hundreds of kilometres out to sea, found a fast, manoeuvring, escorted force and sunk it, for the loss of four aircraft and 18 men.

The G4M1 was the version that fought the opening year, and it was the most-produced single model: 1,172 built including the prototypes. It carried the campaigns in the Philippines, the Dutch East Indies and the long grind over the Solomons and New Guinea, and it took the losses that made the aircraft’s reputation.

A variant that is usually forgotten came first. The G6M1 was a heavily armed escort version of the same airframe, built to fly with bomber formations and defend them with cannon; 30 were completed. Loading a long-range bomber with guns produced an aircraft too heavy to keep up with what it was escorting, and the survivors ended their days as trainers and transports.

The G4M2 was the mid-war standard and the largest family: 429 of the Model 22 series and 713 of the G4M2a Model 24 series, 1,142 in all. It brought uprated Kasei engines, a revised laminar-flow wing, heavier defensive armament and — inevitably — still greater fuel capacity.

What it did not bring was protection. Every improvement went into performance and reach, and the aircraft that met the Hellcat and the Corsair over the Marianas and the Philippines burned exactly as its predecessor had. By this stage the long daylight torpedo run was understood by the crews flying it to be very close to suicide.

The G4M3 finally did the thing the type had needed since 1940. It adopted self-sealing tanks and proper crew armour, redesigned the wing to accept them, and accepted the loss of range that followed — the trade Honjo had refused in 1937 and that the Navy had refused to reconsider for five years of war.

It was completed in 91 examples, out of 2,435 G4M of all types. The fix existed, it worked, and it amounted to under four per cent of production, arriving in the months when Japan no longer had the fuel or the crews to fly what it already had.

The last role was the strangest. The G4M2e was modified to carry the rocket-powered Ohka beneath its belly — a manned flying bomb with a pilot who could not get out, released within sight of the target to make a rocket-assisted dive at a warship. The carrier aircraft, slow and heavy with a winged bomb slung under it, had to fly to within about thirty kilometres of an American fleet to launch at all.

The first operation, on 21 March 1945, went as that arithmetic predicts. The 721st Kokutai sent 18 bombers under Lieutenant Commander Goro Nonaka carrying 16 Ohka, escorted by 30 A6M5. Hellcats intercepted them well short of the fleet, the crews jettisoned their Ohka to try to survive, and every one of the carrier aircraft was shot down. The Ohka pilots of the Jinrai Butai, the Thunder Gods, died in the aircraft carrying them, without reaching a target.

On 18 April 1943 the aircraft’s defining weakness killed the man who had planned Pearl Harbor. American codebreakers read the itinerary of Admiral Isoroku Yamamoto’s inspection flight to Bougainville, and a flight of long-range P-38 Lightnings flew some 700 km at wave height to intercept it. Yamamoto was travelling in a G4M.

The interception worked, and the two Bettys carrying the admiral and his staff were shot down within minutes. A bomber with armour and protected tanks might have survived long enough to matter; this one caught fire and went into the jungle. The Japanese navy lost its most capable strategic mind to an aircraft designed, deliberately, without the weight of anything that would have protected him.

From there the “Betty” fought across the entire Pacific: the Philippines, the Dutch East Indies, the long grinding campaigns over the Solomons and New Guinea, and the defence of the home islands. As Allied fighters and shipboard anti-aircraft fire multiplied, its flammability turned many missions into massacres, and crews came to regard the long daylight torpedo runs as near-suicidal.

The type’s last duty was ceremonial and bleak. In August 1945 two G4Ms painted white with green crosses flew the Japanese surrender delegation to Ie Shima, to be escorted in by American fighters and transferred to a C-54 for the flight to Manila. The Cigar had opened the Pacific war against Force Z and it closed it as a courier.

The Mitsubishi G4M was the backbone of Japanese land-based naval bombing throughout the Pacific war, and in its early months it achieved results — the sinking of capital ships at sea among them — that reshaped naval thinking. No Japanese bomber flew farther or struck at greater reach.

Yet it is remembered above all for the price of that range. Of 2,435 built, the version with protected tanks accounted for 91, and the decision taken in 1937 to trade survivability for distance condemned thousands of airmen to die in burning aircraft. The habit ran through Japanese aircraft design — performance bought with the crew’s margin — and on this type the bill comes to 2,344 aircraft built without protected tanks.

• René J. Francillon, Japanese Aircraft of the Pacific War (Putnam).
• Osamu Tagaya, Mitsubishi Type 1 Rikko ‘Betty’ Units of World War 2 (Osprey, Combat Aircraft).
• Robert C. Mikesh, Japanese Aircraft Interiors 1940–1945 (Monogram) — and his work on the surrender-delegation aircraft.