The Aichi M-Six-A Seiran, the I-Four-Hundred Submarines, and the Floatplane Designed to Strike America from Beneath the Sea

The Aichi M6A Seiran - its name translating from Japanese as Mountain Haze - is the only aircraft ever engineered to fold itself small enough to live inside a submarine’s pressure hull, launch from open ocean, and strike a target hundreds of miles away. One example survives, hanging in a side bay at the Steven F. Udvar-Hazy Center near Washington Dulles. The story behind it is one of the most technically audacious, and ultimately futile, programs of the Second World War.

The I-400 Submarines: The Largest Ever Built

To understand the Seiran, you have to understand the submarines that carried it.

In 1942, the Imperial Japanese Navy began construction on the Sen-toku class - designated by the Americans as the I-400 class - the largest submarines the world had ever seen. Each measured over 400 feet in length, longer than most destroyers of the era, and displaced nearly 6,500 tons when submerged. Nothing in any navy came close.

These were not convoy hunters. They were built to carry aircraft.

Each I-400 class submarine housed three Aichi Seiran floatplanes in a waterproof cylindrical hangar built into the forward pressure hull. That hangar ran nearly 100 feet ahead of the conning tower and measured roughly 12 feet in diameter. Aircraft were stored inside with wings folded, floats folded, and propeller blades flat against the engine cowling - stacked in their cradles like arrows in a quiver.

Engineering the Seiran: Every Component That Could Fold, Did

The Aichi M6A was a two-seat floatplane: pilot forward, observer and gunner aft. Its engine was an Aichi Atsuta, a license-built version of the German Daimler-Benz DB 601, an inverted V-12 producing around 1,400 horsepower. It was a finicky powerplant that demanded careful maintenance - an unusual choice for an aircraft meant to live in a submarine hangar, but it was what Aichi had.

The engineering brief was stark: make it fit, make it fold, make it fast.

Wings folded back against the fuselage. The central float folded up against the belly. Wingtip floats folded flat. The vertical stabilizer hinged down. Propeller blades folded against the nose. Every millimeter of hangar space was accounted for.

Unfolded and buttoned up, the Seiran could reach nearly 300 miles per hour and carry an 800-kilogram bomb or a torpedo. Range was approximately 740 miles - not enormous for a purpose-built strike aircraft, but enough for what Japanese planners had in mind.

From Hangar to Catapult in Under 15 Minutes

The launch sequence was a drilled evolution against the clock.

When the moment came, the crew flooded the foredeck, dragged each aircraft out of the hangar on its wheeled dolly, unfolded the wings and floats, mounted everything in place, and ran through a compressed pre-flight. Practiced relentlessly, the full sequence from hangar to catapult-ready came down to under 15 minutes. In training, sometimes under 10.

A compressed-air catapult on the forward deck flung the aircraft into the sky. The submarine then slipped back beneath the surface before it could be spotted by anything airborne.

The aircraft would fly its mission and then had to locate a submarine somewhere in a featureless ocean - if everything had gone according to plan, if no one was hunting them, if the sea permitted a water landing and recovery. The men who flew the Seiran understood the math on that. They flew anyway.

The Original Target: The Panama Canal

The concept, developed in the early 1940s, centered on a single strategic objective: the Panama Canal.

The logic was clear. The Canal was America’s logistical artery between oceans. Destroy the locks and you cripple the U.S. Navy’s ability to shift forces between theaters. You don’t defeat America that way - but you slow the war machine and buy time while Japan consolidates Pacific gains.

Two submarines carrying six aircraft between them could approach from the Pacific entirely undetected. The Seirans would rise from the sea in darkness, drop their bombs on the locks, and vanish before anyone properly understood what had happened. No carrier would appear on the horizon. No conventional air defense had been designed to anticipate an attack originating from beneath the ocean.

The I-400 class was designed specifically for that reach.

Operation Storm: The Mission That Never Launched

The first I-400 submarines were not commissioned until late 1944. A program originally intended to produce 18 submarines had been gutted by resource shortages, American submarine attacks on Japanese shipping, and bombing of Japanese industrial facilities. Only three were ever completed.

By spring 1945, the Panama Canal mission had been quietly set aside. American forces had swept across the Central Pacific, and the strategic situation had shifted too far for a Canal strike to change much. But Japanese planners had identified a new target.

At Ulithi Atoll in the Western Caroline Islands, the United States Navy had assembled a concentration of naval power on an almost incomprehensible scale: carriers, battleships, cruisers, destroyers, fleet oilers - the full logistical apparatus of the drive toward the Japanese home islands. All of it moored in rows in a lagoon.

Operation Storm called for I-400, I-401, and I-13 to launch their Seirans at night. The aircraft would transit at low altitude to avoid radar and arrive over Ulithi at dawn, striking carriers at anchor. Some historical accounts suggest the aircraft were to be marked with American military insignia to delay identification - if true, a violation of the laws of war. Whether that plan was finalized or abandoned in the weeks before launch, the historical record is genuinely contested.

What is certain: in August 1945, the submarines were already at sea and the aircrew were working through final preparations when word arrived. Japan had surrendered.

The End: Aircraft into the Sea

Japanese naval command issued orders. Seiran aircraft aboard the submarines were to be destroyed. The submarines were to surface and comply with Allied direction.

For the aircrew, the reaction was something close to grief. They had lived inside steel tubes for months. They had drilled the folding and unfolding sequence until their hands knew it without their minds having to engage. They had mentally rehearsed the mission so many times that the target felt more real than the world outside.

Some crews painted rising sun markings on their Seirans before the aircraft went over the side - a final gesture for machines that never flew in anger. Then the aircraft went into the Pacific, and the ocean closed over them.

Why the Americans Scuttled the I-400 Submarines

I-400 and I-401 were escorted under guard to Pearl Harbor, where American naval officers could barely believe what they were looking at. Submarines this large. Cylindrical aircraft hangars in the pressure hull. Catapults on the foredeck. Range figures that seemed impossible. Engineers documented everything.

Surviving Seiran aircraft that had already come stateside were taken to Patuxent River Naval Air Station in Maryland for evaluation. American test pilots flew them, briefly and carefully.

Then Cold War politics intervened.

The Soviet Union, nominally an Allied power, was entitled under Japan’s surrender terms to inspect captured Japanese military equipment. American naval planners refused to allow Soviet engineers access to the I-400 submarines - not the hangar design, not the range capability, not the catapult systems.

So in early 1946, rather than permit Soviet inspection, the United States Navy scuttled both I-400 class submarines in deep water southwest of Oahu. They remain there today, under several thousand feet of water. Japanese research expeditions have located both vessels in recent decades.

The Restoration and What Survives

The Smithsonian Institution acquired one Aichi M6A Seiran through the chain of wartime aircraft custody. By the 1950s it was in Smithsonian storage - missing components, in varying states of decay.

The restoration took decades of interrupted, underfunded, determined work. There were almost no surviving factory drawings, no production manuals to pull off a shelf, and essentially no one left alive who had built or maintained one operationally. Smithsonian staff and volunteers worked from photographs, captured Japanese technical documents, and direct measurement of the airframe itself, fabricating parts where originals could not be found.

The engine was preserved in static condition, not restored to running. This is a display aircraft, not a flying one. But every major system is present. The folding mechanisms work. The lines are right.

When the Udvar-Hazy Center opened in 2003, the Seiran was among its centerpiece artifacts - displayed in the markings it would have carried for the Ulithi mission.

The Concept That Outlived the War

The Seiran never dropped a bomb in combat. It never fired its guns at a target. Its war began and ended in a submarine hangar. The aircraft that came closest to the fight were pushed into the sea by their own crews on the day the war ended.

But the idea the Seiran embodied did not die with it.

The United States Navy deployed submarine-launched Regulus cruise missiles in the 1950s. The Tomahawk missile carried that logic into the modern era. The fundamental proposition - that a submarine can reach out and strike a distant target with a flying weapon from a position that is nearly impossible to locate - is woven into modern strategic naval thinking.

Japan got there with a floatplane and a cylindrical hangar in 1942.

Key Takeaways

The Aichi M6A Seiran was engineered to fold entirely - wings, floats, vertical stabilizer, propeller blades - and fit inside a 12-foot diameter submarine hangar, then deploy and launch in under 15 minutes.
The I-400 class submarines that carried it were the largest in the world: over 400 feet long, displacing nearly 6,500 tons submerged, each capable of carrying three Seirans.
The original target was the Panama Canal; the final planned mission, Operation Storm, targeted the assembled U.S. fleet at Ulithi Atoll and was aborted by Japan’s surrender in August 1945.
To prevent Soviet inspection, the U.S. Navy scuttled both I-400 submarines southwest of Oahu in early 1946; both have since been located by Japanese research expeditions.
One Seiran survives, restored over decades by the Smithsonian and on display at the Steven F. Udvar-Hazy Center in the markings of its intended final mission.