Soon after the advent of aircraft and submarines during World War I, the combined operation of those two weapons were considered by many countries. However, it was only the Imperial Japanese Navy that could put it to practical use. At the outbreak of the Pacific War, many large-sized submarines of the Imperial Japanese Navy were equipped with catapult and small observation-type aircraft desiged to attack harbors and to bomb the U.S. mainland. Acknowledging the success of former attacks by the Imperial Japanese Navy, it decided to plan surprise attack missions to the Panama canal by special submarineborne aircraft at the beginning of 1942. The plane was the Aichi M6A1 Seiran, designed exclusively fot this one exceptional mission. It was in May 1942 when the Aichi Aircraft Company received specifications to develop and produce a special attack bomber. At first, the use of the then new Suisei bomber with some modifications was proposed, but the conversion was found impractical
because of the difficulty to have interchangeability of major assemblies. On November 1943, the first prototype made its maiden flight. Undergoing several improvements, the Aichi Aircraft Company received an order for limited production from the Navy in May 1944. The engine was a 12-cylinder liquid-cooled Atsuta 32, which was based on the German Daimler-Benz DB603. A 12.7mm, type-2 flex mounted machine gun was equipped at the rear, and torpedo or a 250kg/800kg bomb could be carried. The Seiran was to be carried by the I-400 class submarine in the hanger tube of 4.2m in diameter and 30.5m in length. The hanger tube was capable of holding three Seirans. They were stored on catapult launching cars with armaments for quick ejection after surfacing. To cope with the space limitation, the Seiran was designed to conserve space by folding the wings. The wings were pivoted on the main spar where it joined the fuselage. By rotating the leading edge downward, the wing could lie back
flat against the sides of the fuselage. The outer portions of each stabilizer and elevator hinged at 90cm from the fuselage centerline and folded downward. Vertical clearance was obtained by folding the tip of the fin to the right. 28 Seirans including a Nanzan, its ground-take-off-and-landing equivalent, were produced in total by 1945, and submersible carriers for them, the I-400 and I-401, were completed almost simultaneously. Having the target changed from the Panama canal to the U.S. Navy air craft carriers staged at Ulithi Atoll due to the change in priorities the war demanded, the 1st Submarine Flotilla including the I-400 and I-401 with three Seirans each departed Japan on 23 July 1945 for their first and last mission. On 15 August, the flagship I-401 monitored a radio message from headquarters, informing them of Japan"s surrender and the flotilla was ordered to return to the nearest port in Japan. Thus the chance to prove the Seiran"s worth was missed forever.