The Aichi M6A Seiran (晴嵐, "Clear Sky Storm"[1]) is a submarine-launched attack floatplane designed for the Imperial Japanese Navy during World War II.

Aichi was already manufacturing under license, the D4Y1 Suisei (Judy), a relatively small single-engined carrier dive bomber with exceptionally clean lines and high performance.

Detailed engineering studies commenced in an effort to modify the Suisei for use aboard the I-400 submarines but the difficulties in doing so were eventually judged insurmountable and a completely new design was initiated.

The original specification dispensed with a traditional undercarriage but it was later decided to fit the aircraft with detachable twin floats to increase its versatility.

A well-trained crew of four men could roll a Seiran out of its hangar on a collapsible catapult carriage, attach the plane's pontoons and have it readied for flight in approximately 7 minutes.

In order to aid pilot conversion to the Seiran, two examples of a land based trainer version fitted with a retractable undercarriage were built.

[16] The first mission of the Seiran squadron, which was named the Shinryuu Tokubetsukougeki-tai (神龍特別攻撃隊, meaning "God-Dragon Special Attack Squad")[17] was to be a surprise air strike on the Gatun locks of the Panama Canal, to cut the main supply line for US forces in the Pacific.

When the force was finally ready to set off on their mission against Panama, Japan's increasingly desperate situation led to a change in plan, with the target for the attack, called Operation Hikari (Splendour), being switched to the American base at Ulithi Atoll where forces, including aircraft carriers, were massing in preparation for attacks on the Japanese Home Islands.

[18][19][20] All six Seirans on board the two submarines, having been disguised for the operation as American planes in violation of the laws of war,[10] were catapulted into the sea with their wings and stabilizers folded (for the I-401) or pushed overboard (for the I-400) to prevent capture.

The Seiran was surrendered to an American occupation contingent by Lt. Kazuo Akatsuka of the Imperial Japanese Navy, who ferried it from Fukuyama to Yokosuka.