Aichi chief engineer, Toshio Ozaki, designed the M6A1 Seiran to fulfill the requirement for a bomber that could operate exclusively from a submarine. Japanese war planners devised the idea as a means for striking directly at the United States mainland and other important strategic targets that lay thousands of kilometers from Japan. To support Seiran operations, the Japanese developed a fleet of submarine aircraft carriers to bring the aircraft within striking distance. No Seiran ever saw combat but the Seiran/submarine weapons system represents an ingenious blend of aviation and marine technology.
Japan was already operating reconnaissance aircraft from submarines before the United States entered World War II. One of these airplanes actually bombed American soil. On September 9, 1942, a Yokosuka E14Y1 GLEN (Allied codename) reconnaissance floatplane launched by catapult from the submarine I-25 and dropped four improvised phosphorus bombs into a forest on the Oregon coast. Five months earlier, the Japanese Navy issued orders to build a new series of submarine aircraft carriers called the I-400 class. Navy planners envisioned a large fleet but eventually only three were completed, I-400 through I-402. The three ships in this class were the largest submarines ever built until the "USS Lafayette" sailed in 1962. An I-400 boat displaced 5,970 metric tons (6,560 tons) submerged and it cruised at 18.7 knots surfaced. These ships could travel 60,000 km (43,000 mi) carrying three Seirans in waterproof compartments. A class of smaller Japanese submarines called the AM class was also modified to carry two Seirans.
Soon after commencing the I-400 program, the Navy directed Aichi to develop the Prototype Special Attack Aircraft M6A1. Chief engineer Ozaki confronted an ambitious challenge: develop an aircraft to haul a 250 kg (400 lb) bomb, or an 800 kg (1,288 lb) bomb or torpedo, and fly at least 474 kph (294 mph) with jettisonable floats in place, or 559 kph (347 mph) without floats. The navy also stipulated that assembling and launching the three M6A1s should require no more than 30 minutes. To fit inside a 3.5 m (11 ft 6 in) diameter, cylinder-shaped hangar, Ozaki designed the main wing spar to rotate 90° once the deck crew removed the two floats. After rotating the wings, the crew folded them back to lie flat against the fuselage. About 2/3 of the each side of the horizontal stabilizer also folded down, likewise the tip of the vertical stabilizer. Deck crews stored the floats and their support pylons in separate compartments.
Aichi completed the first prototype in October 1943 and started flight tests in November. The second prototype joined the test program in February 1944. The Navy was so pleased with the initial results that it ordered production to start even before Aichi delivered the remaining prototypes, and two land-based M6A1-K Nanzan trainers. However, progress virtually stopped after a major earthquake severely disrupted the production line in December 1944. Boeing B-29 bomber raids further disrupted the project. As the war deteriorated in March 1945, the Navy curtailed the submarine program. The first I-400 was finished on December 30, 1944, and the I-401 followed a week later. But I-402 was converted into a submarine fuel tanker and work ended on I-404 and 405. With the submarine fleet now reduced, the Navy required fewer Seirans so this program was also curtailed. Using parts on hand, Aichi eventually built 26 Seirans (including prototypes) and two Nanzan trainers.
Navy leaders organized the 1st Submarine Flotilla and 631st Air Corps and placed Captain Tatsunoke Ariizumi in command of both units. The combined force consisted of the submarine carriers I-400 and I-401, two AM class submarines, the I-13 and I-14, and 10 Seiran bombers. During sea trials, the units practiced hard to reduce the assembly time for the Seiran. Eventually, the crews could launch three aircraft (albeit without floats) in less than 15 minutes! There was a major drawback for without floats, the Seiran pilot could not safely land on the water. His only option was to ditch the bomber near the submarine and await rescue. The aircraft would obviously be lost.
It was perhaps the most ambitious strategic target selected during World War II. Japanese Navy planners chose to strike the locks of the Panama Canal using the 631st Air Corps embarked aboard the 1st Submarine Flotilla. Planners assigned ten Seirans to strike the Gatun Locks with six torpedoes and four bombs. The pilots studied a large-scale model of the lock system and memorized important features of the canal, just as their predecessors did before attacking Pearl Harbor. During these preparations, the Japanese decided to strike first at the U. S. Navy fleet anchored at Ulithi Atoll. On June 25, 1945, Ariizumi received orders for Operation Hikari. This plan required six Seirans and four Nakajima C6N1 MYRT reconnaissance aircraft (see NASM collection). The I-13 and I-14 would carry two MYRTs each and offload them at Truk Island. The MYRT pilots would take off and scout the American fleet at Ulithi, relaying target information to the Seiran crews. The six Seirans would carry out kamikaze attacks on the most important targets-American aircraft carriers and troop transports.
Trouble dogged the entire operation. The I-13, with two MYRTs aboard, was damaged by air attacks then sunk by a U. S. destroyer. The I-400 missed a crucial radio message from Ariizumi's flagship and proceeded to the wrong rendezvous point. On August, 16, 1945, Ariizumi's flotilla received word that the war was over. They were ordered to return to Japan and scuttle the aircraft. The I-400 crew punched holes in floats and pushed the Seirans overboard as sailors aboard I-401 catapulted their M6A1s into the sea.
The National Air and Space Museum M6A1 was the last airframe built (serial number 28) and today it remains the only extant Seiran. Imperial Japanese Navy Lt. Kazuo Akatsuka ferried this Seiran from Fukuyama to Yokosuka where he surrendered it to an American occupation contingent. The aircraft was shipped to United States, then periodically displayed at Naval Air Station Alameda, California, until the U. S. Navy transferred the aircraft to the Paul E. Garber Facility in Silver Hill, Maryland. It arrived in November 1962 but remained in storage outdoors for 12 years until in-door display/storage space became available.
Restoration work began on the floatplane in June 1989 and ended in February 2000, thanks to the outstanding work of a team of staff experts, many volunteers, and several Japanese nationals working at Garber and in Japan. No production drawings survive and the team conducted exhaustive research into how various aircraft systems operated in order accurately reconstruct a number of missing components. They found interesting design features built into the Seiran that ranged in engineering quality from the ingenious to the seemingly absurd. This artifact also bore witness to the difficult working conditions that plagued the Japanese aviation industry at the end of the war. Quality and workmanship were seriously lacking because of extensive damage to equipment and factories and the lack of skilled, professional workers (many were high school students). A metal flap bore damage-probably the result of a bombing raid-hastily covered with fabric patches. They found the interior of fuel tanks contaminated with paper documents. Basic fit and alignment of parts was also poor in many places. Someone, possibly a Japanese student, scratched a complete English alphabet inside one wing panel. Technicians found more graffiti in various areas on the airframe.
Craftsmen were surprised to find no evidence that the pilot could jettison the floats in flight, contrary to claims by the designer. Aichi may have deleted this feature near the end of the M6A1 production run.
