In 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.
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