日本海軍が開発した水上攻撃機。
伊400型潜水艦を母艦として出撃し、対象に急降下爆撃を行うという、世界にも類を見ない潜水艦載攻撃機です。
普段はフロートを外して翼を折りたたみ、潜水艦に設置された筒状の格納庫に身を潜めています。また出撃までに要する時間は約3分と、複雑な機構で収納されている割には驚異的なスピードです。
800kgまでの爆弾もしくは魚雷を装備し、潜水艦から射出後爆撃もしくは雷撃を行い、フロートで着水して回収という形をとりました。また潜水艦には予備の爆弾や魚雷も保管してあった為、場合によっては再出撃も可能と、きちんと艦載機としての体をなしていたようです。

The Aichi M6A Seiran was an attack seaplane developed by the Imperial Japanese Navy.
Deployed from I-400 class submarines, it operated by dive-bombing at the target under attack and was a 'submarine-launched dive/ torpedo bomber' of the type that the world had never seen before.
The floats were usually removed and the wings folded up for concealment on the cylindrical hangar located on the submarine. In addition to this, it took three minutes to ready it for deployment. Considering the complex mechanisms used to store it away, this was an incredibly short amount of time.
Equipped with up to 800kg of bombs or torpedoes, after being launched from the submarine and carrying out its bomb or torpedo attack, it would land in the water with its pontoons and adopt the position for recovery. Also, since the submarine contained reserve bombs and torpedoes, it was possible for the Seiran to be sent out again and to perform as a carrier-based aircraft when the occasion called for it.