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 the G4M — a sleek, cigar-shaped twin-engined bomber whose deep, rounded fuselage gave it the crew nickname “Hamaki”, the Cigar. To reach the colossal range the Navy wanted, Honjo's team made a fateful choice: they filled the wings with enormous, unprotected integral fuel tanks and stripped out armour and fire suppression. The G4M could fly astonishing distances — but it would carry that decision as a fatal flaw for the rest of its career.

Powered by two Mitsubishi Kasei radials, the G4M combined long range with a useful turn of speed and a heavy offensive load — a single aerial torpedo or up to a tonne of bombs. Defensive armament was respectable for its day, with machine guns and a 20mm cannon in the tail, and the type handled well in the air.

But the unprotected wing tanks made it terrifyingly vulnerable. A few well-placed rounds were enough to set the fuel ablaze, and Allied fighter pilots quickly learned how easily the big bomber would burn. They christened it the “one-shot lighter”, and Japanese crews, with grim humour, called it the “Flying Lighter”. Losses among the long, slow over-water missions mounted alarmingly once Allied fighter strength grew.

Mitsubishi spent the rest of the war chasing a cure. The G4M2 introduced more powerful engines, a revised laminar-flow wing and still greater fuel capacity, but left the protection problem largely unsolved. Only the late G4M3 finally adopted self-sealing tanks and crew armour at the cost of range — and by then it was far too late to matter.

The original G4M1 bore the brunt of the early-war campaigns. The much-improved G4M2, with its new wing, uprated Kasei engines and heavier defensive armament, became the most-produced version and the standard naval bomber of the middle war years.

The G4M3 at last brought self-sealing tanks and proper armour protection, addressing the type's notorious flammability, but appeared only in small numbers near the war's end. A specialised conversion, the G4M2e, was modified to carry the rocket-powered Ohka suicide flying bomb slung beneath its belly. About 2,400 G4Ms of all types were built.

The G4M announced itself in spectacular and terrible fashion. On 10 December 1941, land-based naval bombers — G4Ms among them — found and sank the British capital ships HMS Prince of Wales and HMS Repulse off Malaya, the first time fast battleships at sea had been destroyed by air power alone. It was a demonstration of exactly the over-water striking reach the Navy had sought.

That reach had been bought at a terrible price. To fly so far carrying a torpedo, the designers had left out armour and self-sealing fuel tanks, filling the wings with unprotected petrol. Allied pilots soon nicknamed the “Betty” the “one-shot lighter” for the ease with which it caught fire, and its own crews knew that a single burst in the right place could turn the aircraft into a torch.

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.

Two moments fixed the G4M in history. In April 1943, American P-38s — acting on decrypted signals — shot down the two “Betty” bombers carrying Admiral Isoroku Yamamoto over Bougainville, killing Japan's foremost naval strategist. And in August 1945, two G4Ms painted white with green crosses carried the Japanese surrender delegation to Ie Shima — the Cigar's final, sombre duty.

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. The decision to trade protection for distance condemned thousands of airmen to die in burning aircraft, and the “one-shot lighter” became a byword for the Japanese tendency to sacrifice survivability for performance. Capable, far-ranging and tragically fragile, the G4M embodied both the ambition and the fatal compromises of Japanese naval air power.

• 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).