Aichi M6A The was a submarine - launched attack floatplane designed for the Imperial Japanese Navy during World War II . It was intended to operate from I - 400 class submarines whose original mission was to conduct aerial attacks against the United States . Design and development . From the late 1920s , the Imperial Japanese Navy had developed a doctrine of operating floatplanes from submarines to search for targets . In December 1941 , however , Commander - in - Chief of the Japanese Combined Fleet , Admiral Isoroku Yamamoto , proposed constructing a large fleet of submarine aircraft carrier s ( also designated STo or " sen - toku " — special submarine ) whose purpose was to mount aerial attacks against American coastal cities . The submarines would surface to launch their aircraft by catapult , submerge to avoid detection , then surface again to retrieve the aircrews who would ditch their planes nearby . By June 1942 , the plan was to build a fleet of eighteen such subs . This was later cut to nine , then five and finally just three as Japan ' s wartime fortunes declined . To equip the submarine aircraft carriers , the Imperial Japanese Navy Air Service requested that Aichi design a folding attack aircraft with a range of and a speed of . 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 Susei for use aboard the I - 400 submarines but the difficulties in doing so were eventually judged insurmountable and a completely new design was initiated . Aichi ' s final design , designated AM - 24 by Aichi and given the military designation M6A1 , was a two - seat , low - winged monoplane powered by a Aichi AE1P Atsuta 30 engine ( a licence - built copy of the Daimler - Benz DB 601 liquid - cooled V12 engine ) . 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 . If conditions permitted , these would allow the aircraft to alight next to the submarine , be recovered by crane and then re - used . The floats could be jettisoned in flight to increase performance or left off altogether for one - way missions . The " Seiran ' s " wings rotated 90 degrees and folded hydraulically against the aircraft ' s fuselage ( with the tail also folding down ) to allow for storage within the submarine ' s diameter cylindrical hangar . Armament was a single torpedo or an equivalent weight in bombs . One Type 2 machine gun was mounted on a flexible mounting for use by the observer . As finalised , each I - 400 class submarine had an enlarged watertight hangar capable of accommodating up to three M6A1s . The " Seirans " were to be launched via a compressed - air catapult mounted on the forward deck . 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 shorten the launching process and eliminate the need for time - consuming engine warm - ups , the " Seirans " were to be catapulted from a cold start . This necessitated heating the engine oil for each plane to approximately in a separate chamber and pumping it , as well as hot water , through the engine just prior to launch while the planes were still in the hangar . In this way , the aircraft ' s engine would be at or near normal operating temperature immediately upon getting airborne . The idea was borrowed from the Germans who planned on using a similar launch method for the aircraft of their unfinished carrier " Graf Zeppelin " . The first of eight prototype " Seirans " was completed in October 1943 , commencing flight testing in November that year . A problem with overbalance of the auxiliary wings was eventually solved by raising the height of the tail fin . Further testing was sufficiently successful for production to start in early 1944 . In order to aid pilot conversion to the " Seiran " , two examples of a land based trainer version fitted with a retractable undercarriage were built . These were given the designation M6A1 - K " Nanzan " ( " Southern Mountain " ) . Besides the difference in landing gear , the vertical stabilizer ' s top portion , which was foldable on the " Seiran " , was removed . Operational history . The first production examples of the " Seiran " were completed in October 1944 . Deliveries were slowed by an earthquake near Nagoya on 7 December 1944 , and by an American air - raid on 12 March 1945 . Construction of the STo submarines was stopped in March 1945 , after two submarine aircraft carriers had been completed and a third finished as a fuel tanker . These were supplemented by two smaller Type AM submarine s , originally designed as command submarines carrying reconnaissance floatplanes , but capable of carrying two " Seirans " . Owing to the reduced carrier submarine force , production of the " Seiran " was halted , with a total of 28 completed ( including the prototypes and the M6A1 - Ks ) . The new submarines and aircraft were assigned to the 1st Submarine Flotilla , comprising the two STo submarines , the " I - 400 " and the flagship " I - 401 " , each carrying three " Seirans " together with two type AMs , the " I - 13 " and " I - 14 " . The 1st Submarine Flotilla commenced training with the " Seirans " in January 1945 , the crews gradually learning how to handle the submarines and aircraft . Launching all three " Seirans " took longer than expected : 30 minutes if floats were fitted , although this could be reduced to 14 . 5 minutes if the floats were not used . The first mission of the " Seiran " squadron , which was named the 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 , however , 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 . The flotilla departed Japan on 23 July 1945 and proceeded towards Ulithi . However , on 16 August , the flagship " I - 401 " received a radio message from headquarters , informing them of Japan ' s surrender and ordering them to return to Japan . All six " Seirans " on board the two submarines , having been disguised for the operation as American planes in violation of the laws of war , 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 . Survivors . A single M6A1 has been preserved and resides in the Udvar - Hazy Center of the Smithsonian ' s National Air and Space Museum . It is located in the Washington , DC suburb of Chantilly , VA near Dulles International Airport . 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 . The US Navy donated it to the Smithsonian Institution in November 1962 . Restoration work on the " Seiran " began in June 1989 and was completed in February 2000 .