This oft-forgotten Aichi aircraft design was classified as an attack floatplane, designed exclusively for the launching from Imperial Japanese Navy (IJN) I-400-class submarines. The IJN plan was to carry these aircraft in water-tight containers until arriving at a launch waypoint. The vessel would then surface, the aircraft readied, and then launched towards a target of opportunity. To help with storage aboard the submarine, the Aichi M6A Seiran ("Mountain Haze") was designed with wings that rotated and lay flat along the fuselage sides as well as tail surfaces that folded at their tips. Overall, the aircraft was largely conventional, featuring floatplanes for water-borne landings and a crew of two to spread the workload. Power was from an Aichi Atsuta Type 31 liquid-cooled V12 inline piston engine of 1,400 horsepower driving a three-blade propeller at front. Armed with a 13mm machine gun, the M6A was also to carry an offensive load of 1 x Type 91 torpedo or up to 1,875lbs of drop ordnance.
Aichi designed, developed, and produced their M6A series from 1943 to 1945 - the final year of the war. The type was being prepared for a surprise attack on the Panama Canal , a vital waterway for U.S. Naval operations in the Pacific. However, Allied advanced in the Pacific Theater, as well as the dropping of the two Atomic bombs on Japan forced an unconditional surrender for the Empire of Japan. As such, the aircraft did not see combat service in the war. Production was also disrupted due to the unrelenting Allied bombing campaign of the Japanese mainland which led to only 28 examples manufactured.
Eight base M6A1 prototypes preceded the eighteen M6A1 Seiran production models. A land-based derivative, known as the M6A1K "Seiran Kai", was intended for pilot and these would have been deployed with a wheeled undercarriage and of the tail-dragger arrangement for land-based service - losing the floatplane equipment. Only two prototypes were built. The M6A2 was a proposed platform that existed in a sole prototype - this outfitted with a Mitsubishi Kinsei MK8P 1,560 horsepower engine driving a constant speed propeller.
Performance specifications for the Seiran included a maximum speed of 295 miles per hour, a cruise speed of 185 miles per hour, a range of 740 miles and a service ceiling of 32,500 feet.
A sole, intact, Seiran floatplane aircraft is displayed by the Smithsonian National Air and Space Museum, Steven F. Udvar-Hazy Center in Washington, D.C. This aircraft represents the last production model completed. It was surrendered to U.S. occupation forces at the close of the war.