I-400-class Submarine

The I-400-class submarines were among the strangest and most ambitious warships of the Second World War. At 122 metres long and displacing over 6,600 tonnes submerged, they were by far the largest submarines ever built — a record they would hold until the nuclear-powered boats of the 1960s. But their size was not an end in itself: each was built around a hangar, because each was designed to carry aircraft.

Inside a great watertight cylinder faired into the hull, an I-400 stowed three folding-wing floatplane bombers, which it could surface, assemble, launch by catapult and — the plan went — recover again. It was, in effect, a submersible aircraft carrier: a boat that could cross oceans unseen and strike targets thousands of kilometres beyond the reach of any conventional submarine. Nothing quite like it had been built before, and nothing quite like it has been built since.

The concept came from the top. Admiral Isoroku Yamamoto, architect of the Pearl Harbor attack, wanted a way to carry the war to the American homeland itself — to bomb cities on the US mainland, or to strike a blow of real strategic weight. The target that came to obsess the planners was the Panama Canal: destroy its great Gatún locks, and the waterway through which American warships and supplies flowed between the oceans would be blocked for months.

No aircraft could reach Panama from Japan, and no carrier could get close. The answer was a submarine that could — one large enough to carry its own bombers around the world if need be. In 1942 the navy ordered a class of these giants, the Sen-Toku (“special submarine”) type: eighteen boats were planned, each a mobile airfield beneath the sea.

To carry three aircraft and the fuel to range across the globe, the design grew to extraordinary size. Its pressure hull was built as a figure-of-eight in cross-section — two cylinders side by side for strength and stability — topped by the long cylindrical aircraft hangar, with a compressed-air catapult running along the foredeck. Four diesels drove it at nearly 19 knots on the surface.

Its defining quality was range. An I-400 could steam some 37,500 nautical miles without refuelling — enough to circle the earth one and a half times — reaching any coast on the planet. It was heavily armed for a submarine, too, with eight torpedo tubes, a 140 mm deck gun and a battery of 25 mm anti-aircraft cannon. But the boats were also vast, complex and hard to handle, and slow and clumsy once submerged.

The weapon these submarines existed to deliver was the Aichi M6A Seiran, a sleek floatplane bomber designed specifically to fold up and live inside a submarine. Its wings rotated and folded flat against the fuselage and its tail surfaces hinged down, so that the whole aircraft could be stowed in the cramped hangar and made ready to fly by a trained crew in minutes.

Launched off the catapult carrying a bomb or a torpedo, the Seiran gave the I-400 a reach no gun or torpedo could match. Three of them, striking together out of an empty ocean, were meant to be enough to cripple a target as formidable as the Panama locks. Boat and aircraft were conceived as a single weapon system — each useless without the other.

By the time the first boats were ready in early 1945, the war had turned decisively against Japan, and the ambitions for the class had shrunk with it. Yamamoto was dead; the Panama raid was judged too distant a prize for a nation fighting for survival, and the target was changed to something closer and more urgent — the great American fleet anchorage at Ulithi Atoll, crowded with the carriers preparing to invade Japan itself.

The operation, codenamed Arashi (“Storm”), would send the submarines and their Seiran to fall on the anchored fleet in a surprise dawn strike. The two operational boats, I-400 and I-401, put to sea in the summer of 1945 and began the long approach across the Pacific toward their target.

They never arrived. On 15 August 1945, with the boats still at sea, Japan surrendered. The strike was cancelled, and the crews were ordered to destroy their weapons: the Seiran were catapulted into the sea without their pilots, and the torpedoes fired off into the empty ocean, so that the submarines’ cargo would not fall into enemy hands.

Only three of the eighteen planned boats had been completed — I-400, I-401 and I-402 — and none had fired a shot in anger. The most extraordinary submarines of the war had been finished just too late to be used, their audacious mission overtaken by the collapse of the country that built them.

The surrendered boats were a revelation to the Americans who boarded them — decades ahead of anything in the US inventory. They were taken to Hawaii and studied intently, and then, as the wartime alliance curdled into the Cold War and a Soviet inspection team asked to see them, the US Navy chose to put the secret beyond reach: in 1946 the giant submarines were scuttled off Oʻahu, sent to the bottom by American torpedoes.

There they lay forgotten until, decades later, undersea expeditions rediscovered their wrecks on the seabed off Hawaii. The I-400s never launched their strike, and as a weapon they proved a technological dead end. But as an idea — a submarine that carried its own air force across the world’s oceans — they were unlike anything else afloat, and they remained the largest submarines ever built for a generation, until the coming of the nuclear age.

• Carl Boyd & Akihiko Yoshida, The Japanese Submarine Force and World War II (Naval Institute Press).
• Henry Sakaida, Gary Nila & Kōji Takaki, I-400: Japan’s Secret Aircraft-Carrying Strike Submarine (Hikoki Publications).
• John Geoghegan, Operation Storm: Japan’s Top Secret Submarines and Its Plan to Change the Course of World War II (Crown).
• Mark Stille, Imperial Japanese Navy Submarines 1941–45 (Osprey, New Vanguard).