Die Aichi M6A Seiran („Gebirgsdunst“) war ein japanisches Flugzeug aus der Zeit des Zweiten Weltkriegs. Die Werksbezeichnung lautete AM-24.

Sie war ein Tiefdecker mit zwei abwerfbaren zentralen Schwimmern unter dem Rumpf als U-Boot gestützter Sturzbomber (M6A1), der nach Abwurf der Schwimmer im Wasser eine Bauchlandung machen sollte. Später wurde auch eine Version mit einem einziehbaren Fahrwerk als landgestütztes Übungsflugzeug oder Sturzbomber konstruiert, der nach dem Start vom U-Boot an Land landen sollte (M6A1-K Nanzan („Südlicher Berg“)). Von Oktober 1943 bis Juli 1945 wurden 28 M6A1 gebaut, davon waren zwei M6A1-K.

Auffällig ist die elegante stromlinienförmige Rumpfform. Um das Flugzeug in Hangars auf einem U-Boot unterbringen zu können, waren die Tragflächen nach hinten seitlich anklappbar, das Höhenleitwerk konnte zum Großteil nach unten und die Oberkante des Seitenleitwerks zur Seite geklappt werden. Die Schwimmer sollten erst kurz vor dem Einsatz montiert werden. Für dieses Flugzeug wurde kein alliierter Codename vergeben, da dem amerikanischen Geheimdienst die Existenz des Flugzeugs bis zum Kriegsende verborgen blieb.

Ursprünglich sollte die Seiran von großen U-Booten des Typs I-400 aus den Panama-Kanal angreifen, um ihn unpassierbar zu machen. Diese Pläne wurden jedoch aufgrund der Kriegslage beim Stapellauf des ersten I-400 U-Bootes 1944 gestrichen. Eine M6A1 blieb erhalten und gehört heute dem National Air and Space Museum in Washington (D.C.).

The only extant copy of the Aichi M6A1 Seiran is on display at the Udvar-Hazy annex of the National Air and Space Museum.

The Seiran was built to meet a requirement for a bomber that could operate exclusively from a submarine. Japanese war planners devised the idea as a means for striking directly at the United States mainland and other important strategic targets that lay thousands of kilometers from Japan, including the locks of the Panama Canal.

To support Seiran operations, the Japanese developed a fleet of submarine aircraft carriers to bring the aircraft within striking distance. No Seiran ever saw combat but the Seiran/submarine weapons system represents an ingenious blend of aviation and marine technology.

Japan was already operating reconnaissance aircraft from submarines before the United States entered World War II. One of these airplanes actually bombed American soil. On September 9, 1942, a Yokosuka E14Y1 GLEN reconnaissance floatplane was launched by catapult from the submarine I-25 and it dropped four improvised phosphorus bombs into a forest on the Oregon coast.

Five months earlier, the Japanese Navy issued orders to build a new series of submarine aircraft carriers called the I-400 class. Navy planners envisioned a large fleet but eventually only three were completed: I-400 through I-402. The five ships in this class were the largest submarines ever built until the SSBN USS Lafayette sailed in 1962.

An I-400 boat displaced 6,560 tons submerged and cruised at 18.7 knots surfaced.

These ships could travel 4,300NM carrying three Seirans in waterproof compartments. Soon after commencing the I-400 program, the navy directed Aichi to develop the Prototype Special Attack Aircraft M6A1. Chief engineer Ozaki confronted an ambitious challenge: develop an aircraft to haul a 400 lb bomb, or an 800 kg (1,288 lb) bomb or torpedo, and fly at least 294KTS with jettisonable floats in place, or 347KTS without.

For stealth, the IJN also stipulated that assembly and launch if three M6A1s should require no more than 30 minutes. To fit inside a 11 ft 6 in-diameter, cylinder-shaped hangar, Ozaki designed the main wing spar to rotate 90* once the deck crew removed the two floats. After rotating the wings, the crew folded them back to lie flat against the fuselage. About 2/3 of the each side of the horizontal stabilizer also folded down, likewise the tip of the vertical stabilizer. Deck crews stored the floats and their support pylons in separate compartments.

Aichi completed the first prototype in October 1943 and started flight tests in November. The second prototype joined the test program in February 1944. The Navy was so pleased with the initial results that it ordered production to start even before Aichi delivered the remaining prototypes, and two land-based M6A1-K Nanzan trainers.

Progress virtually stopped after a major earthquake severely disrupted the production line in December 1944. Boeing B-29 bomber raids further disrupted the project. As the war deteriorated in March 1945, the Navy curtailed the submarine program. The first I-400 was finished on December 30, 1944, and the I-401 followed a week later. But I-402 was converted into a submarine fuel tanker and work ended on I-404 and I-405. With the submarine fleet now reduced, the navy required fewer Seirans so this program was also curtailed. Using parts on hand, Aichi eventually built 26 Seirans (including prototypes) and two Nanzan trainers.

Navy leaders organized t

he 1st Submarine Flotilla and 631st Air Corps and placed Captain Tatsunoke Ariizumi in command of both units. The combined force consisted of the submarine carriers I-400 and I-401, two AM class submarines, the I-13 and I-14, and 10 Seiran bombers. During sea trials, the units practiced hard to reduce the assembly time for the Seiran. Eventually, the crews could launch three aircraft (albeit without floats) in less than 15 minutes.

There was a major drawback, since without floats, the Seiran pilot would be flying a one-way mission, similar to some of the nuclear A-4 missions in the SIOP. The option of preference was to ditch the bomber near the submarine and await rescue.

The most ambitious strategic target selected during World War II was near the end, when IJN planners selected the Gatun Locks of the Panama Canal (on the Caribbean side of Gatun Lake) using the 631st Air Corps embarked aboard the 1st Submarine Flotilla. Planners assigned ten Seirans to strike the locks with six torpedoes and four bombs.

The pilots studied a large-scale model of the lock system and memorized important features of the canal, just as their predecessors did before attacking Pearl Harbor. During these preparations, the Japanese decided to strike first at the U. S. Navy fleet anchored at Ulithi Atoll. On June 25, 1945, Ariizumi received orders for Operation Hikari.

This plan required six Seirans and four Nakajima C6N1 MYRT reconnaissance aircraft. The I-13 and I-14 were planned to carry two MYRTs each and offload them at Truk Island. The MYRT pilots would then take off and scout the American fleet at Ulithi, relaying target information to the Seiran crews. The six Seirans would carry out kamikaze attacks on the most important targets-American aircraft carriers and troop transports.

Trouble dogged the entire operation. The I-13, with two MYRTs aboard, was damaged by air attacks then sunk by a U. S. destroyer. The I- 400 missed a crucial radio message from Ariizumi’s flagship and proceeded to the wrong rendezvous point. On August, 16, 1945, Ariizumi’s flotilla received word that the war was over.

They were ordered to return to Japan and scuttle the aircraft. The I-400 crew punched holes in floats and pushed the Seirans overboard as sailors aboard I-401 catapulted their M6A1s into the sea.

The M6A1 in the collection at Dulles was the last airframe built (serial number 28) and is the only extant Seiran. Allied forces discovered it in the remains of the Aichi factory. The aircraft was shipped to United States, then periodically displayed at Naval Air Station Alameda, California, until the U. S. Navy transferred the aircraft to the Paul E. Garber Facility in Silver Hill, Maryland.