Aichi M6A Seiran

From the late 1920s the Imperial Japanese Navy had a doctrine of operating seaplanes from submarines for scouting purposes. Later, in December 1941, shortly after the attack on Pearl Harbour, the admiral Isoroku Yamamoto, proposed the construction of a large fleet of submarine aircraft carriers with the purpose of performing aerial attacks against American coastal cities. Those submarines would surface to deploy their aircraft, submerge to avoid being detected, a emerge again to retrieve the aircraft that had been previously ditched. In June 1942 the plan was to build a fleet of 18 of such submarines, but the number decreased to nine, then to five and finally to three due to Japan's war fortunes decreased.
To equip such submarine fleet, the Imperial Japanese Navy Air Service requested Aichi to develop an attack aircraft with foldable wings, with a range of 1500km (810nmi) and a speed of 555 km/h (300kn).
Initially Aichi considered adapting the yokosuka D4Y1 Susei (because they were manufacturing it under license) into those new submarines (named I-400 series) as it would be easier. However the dificulties of adapting it were judged insurmountable and therefore a completely new design was initiated.
Aichi's final design, named internally as AM-24, received the military designation of M6A1 was a two-seat, low-winged monoplane powered by a 1050Kw (1410hp) Aichi E1P Atsuta 30 engine which was a licensed copy of the German Daimler-Benz DB 601 liquid-cooled V12 engine. Originally it was expected to carry a traditional undercarriage, but it was decided later to equip it with detacheable twin floats to make it more versatile. The floats could be jettisoned to increase its performance or for one-way missions only. Its wings could be rotated 90º and folded hydraulically against the aircraft's fuselage, with the tail also folding down to allow it to be storaged in the I-400's cylindrical hangar. It could carry a payload of a single 850kg (1870lb) torpedo or its equivalent in bombs under the fuselage and had a defensive 13mm (0.51in) Type 2 machine gun mounted on a flexible position in the observer's position.
Every I-400 was expected to have a sealed watertight hangar capable of accomodating up to three M6A1s which were to be launched from a 26m (85ft) compressed-air catapult mounted on the forward deck. It was expected that the process of rolling the Seirans out of the hangar and make them ready to flight in approximately 7 minutes with a well-trained crew.
In order to make the launching process shorter, the engine warm-up periods needed to be eliminated, so the Seirans should've been catapulted from cold start. Therefore the engine oil was heated up to 60ºC (140ºF) in a separate chamber and pumped, together with hot water through the engine prior to the launch, while the aircraft was still in the hangar. That way the aircraft's engine would be at or near normal operating temperature inmediately upon getting airborne. This system was copied from the Germans who had a similar technique for their Graf-Zeppelin aircraft carrier.
The first Seiran prototype was completed in October 1943 and flew for the first time in November that year. An overbalancing wing problem was detected but it was quickly solved by raising height of the tail fin. Further testing was performed and it was successful enough to make it into production in early 1944. To aid the pilot, two land-based examples were built with retractable undercarriage and received the designation of M6A1-K Nanzan (southern mountains) and lacked the foldable vertical stabilizer's top part.
The first production batches were completed in October 1944 as deliveries were slowed due to an earthquake near Nagoya on 7th December 1944 and by an American air-raid on 12th March 1945. The construction of the submarines was halted in March 1945, after two of them were completed and a third one being repurposed as an submarine fuel tanker. They were supplemented by the smaller Type AM submarines which could also carry up to two Seirans. Due to the scarce production of the I-400 type submarines, the production of the Seiran was stopped with only 28 of them completed, including the prototypes and the M6A1-K.
The new aircrafts were assigned to the 1st Submarine flotilla, which comprised the I-400 and the I-401 (which was the flagship), each of them carrying three Seirans together with the I-13 and I-14, two type AMs submarines. The unit started to train with the Seiran in January 1945 with the crews learning how to handle submarines and aircrafts. It was then discovered that the launching process was longer than expected, 30 minutes if floats were fitted and 14 if floats weren't fitted.
The first mission of the Seiran squadron was to be a surprise sneak attack on the Gatun locks of the Panama canal to cut the main supply line for American forces in the Pacific. However, Japan's desperate war situation, the attack was switched to American base at Ulithi atoll, where forces were massing waiting for the invasion of Japan. The flotilla departed Japan on 23rd July 1945 bound for Ulithi, but on 16th August 1945 the I-401 received a radio message from headquarters informing them about Japanese surrender and ordering them to return to Japan. In order to prevent the aircrafts from being captured (they were painted with USAAF markings to fool the Allied forces) they were either catapulted into the sea or pushed overboard with their wings and stabilizers folded.
However, a single one was saved when its pilot, Lt. Kazuo Akatsuka, of the Imperial Japanese Navy, ferried it from Fukuyama to Yokosuka and surrendered to American forces.

Sources:
1. https://en.wikipedia.org/wiki/Aichi_M6A
2. https://www.militaryfactory.com/aircraft/detail.asp?aircraft_id=445