The Aichi M6A Seiran was a rare late World War II Japanese seaplane bomber with a compact, folding design and high performance. It was designed for operation from the huge I-400 class submarine aircraft carriers, making it the only piloted, submarine-launched, dedicated strike aircraft ever to enter service. No M6A ever saw combat, the type's sole operational mission having been aborted by the end of the Pacific War. Twenty-nine examples were built of which only one survives today, in the Smithsonian National Air and Space Museum in the United States.
The M6A was a low-winged monoplane with a 1,400 horespower liquid-cooled V12 engine, bearing a more-than-coincidental resemblance to the D4Y Suisei carrier-based dive bomber which Aichi was already manufacturing under license. The two man crew consisted of a pilot and an observer sitting behind him who navigated, operated the radio, and fired the plane's defensive armament of a single 13mm machine gun in a flexible mount. The aircraft was launched by catapult and could be reconfigured in the field to use two floats for landing and recovering at sea, or no undercarriage for improved performance at the cost of sacrificing the aircraft and possibly its crew after the mission. It carried either a single 850 kg torpedo or an equivalent weight of bombs.
Perhaps more than any other Axis "wonder weapon", the Seiran combined a complex and imaginative operational concept, impressive technical achievements, and a complete lack of attention to the rational allocation of scarce resources relative to the results it could hope to achieve. It is therefore not surprising that it was a pet project of admiral Yamamoto Isoroku, who was notorious for bypassing staff analysis to pursue his chosen schemes. That said, late-war technologies unavailable to the Japanese meant that the I-400/Seiran combination became an important precursor to the U.S. Navy's experimental 'Loon' and operational 'Regulus' submarine-launched nuclear cruise missiles.
