Immediately after the December 7, 1941 attack on Pearl Harbor, Admiral of the Fleet Isoroku Yamamoto, supreme commander of the Imperial Japanese Navy’s Combined Fleet and the originator of the Pearl Harbor attack strategy, met with his staff to plan the next move. The 1989 book Phantom of Submarine Aircraft Carrier cites an account by one of Yamamoto’s staff officers that the admiral was working on another daring scheme. In the old Japanese organized crime groups, known as the yakuza, there is a gambling term—kenkon itteki, meaning “all or nothing, a one shot.” Yamamoto was no yakuza, but he knew the meaning of the words.
Japan must “put our hands” on the U.S. mainland, he said, adding: “What other means is there than the aircraft-carrying submarine?” And so, just over a month later, on January 13, 1942, the Imperial Japanese Navy’s Kansei Honbu (Ships Command Headquarters) received a request from the Gunreibu (Naval Operations Command) to study the possibility of designing a “40,000-nautical-mile range submarine with a capacity to carry attack aircraft each loaded with a torpedo or 800 kg [1,764 pound] bomb.”
When Yamamoto had been assigned to prosecute Japan’s war against the mighty naval powers of the United States and Great Britain, he considered the task impossible. Still, he had hoped to score a decisive win at Pearl Harbor to gain the upper hand in what would surely be a long and wearing war. Yamamoto knew he had to have another tactical victory if Japan were to win strategically or at least maneuver for an advantageous truce. Submarine-stowed aircraft for clandestine reconnaissance and attack against enemy ships had been built and tested by several seagoing nations as long ago as World War I. But Yamamoto’s concept of using a submarine’s stealth to deliver an airborne attack on strategic land targets was clearly a departure.
Japanese memoirs speculate that Yamamoto’s vision for the kenkon itteki included targets like New York City or Washington, D.C. As planning progressed, however, the navy settled on a more pragmatic target: the Panama Canal, where a single raid, if successful, could prevent the U.S. Atlantic fleet from crossing to the Pacific. An undersea-borne air strike force would launch from the Gulf of Panama, cross Panama to the Caribbean Sea at low altitude, surprise the defenders by approaching from the Atlantic side, and bomb the critical Gatun Locks, which would put the canal out of operation for six months.
In early 1942, both the Ships Command Headquarters’ submarine division and the Air Command Headquarters’ Kugisho (Air Technical Bureau) began working on a submarine carrier and an attack aircraft that could be launched from it. Engineers developed two carrier submarine classes, called sen-toku, or special-type sub: One, an I-13–class sub, displaced 3,603 tons on the surface and had a range of 21,000 nautical miles. The larger, I-400, was 400 feet long, displaced 5,223 tons surfaced, and had a 37,500-nautical-mile range. Each was designed to carry two aircraft in a hangar tube built over the hull, but the I-400 was revised to carry three.
The aircraft designated M6A1 was one of the 17-shi (experimental aircraft initiated in the 17th year of Showa—1942) and was eventually named Seiran. (“Seiran” translates as sei, meaning “clear sky,” and ran, meaning “storm,” thus: “storm from the clear sky.”) In a chapter he wrote in Development and Records of Famous Aircraft the Seiran’s chief test pilot, Tadashi Funada, explained how he conceived of the name for the airplane: “An instruction came from the Kaigun Kouku Honbu (Naval Air Command Headquarters) to think up an appropriate name for the M6A1 Prototype Aircraft. I contemplated adding to the stealth of the submarine the ‘limited visibility maneuvering ability (instrument flying skill)’ of the floatplane pilots. As a metaphor for suddenly appearing out of the fog—like a Ninja—I thought of the name ‘Seiran’ from ‘Seiran of Awazu.’ ” This is the title of a series of woodprints, by an 18th century master craftsman named Hiroshige, typically depicting mountain haze rising, presumably after a storm in Awazu, a picturesque town.
The design and construction were assigned by the Kugisho on May 15, 1942, to the Aichi Kouku (Aircraft) Company of Nagoya, which had already built several floatplanes for the navy. Basic design studies for the Seiran consumed Aichi for the rest of 1942, and the first prototype was completed in November 1943, followed by seven more. During 1944, flight tests at the Kugisho in Yokosuka were conducted and developmental changes made. In January 1945, the aircraft was ready for operational use.
Leadership of the project was entrusted to Aichi’s chief designer, Toshio Ozaki, and the Kugisho’s chief test pilot, Lieutenant Commander Tadashi Funada. The effort to design a large, high-performance attack aircraft that would fold to fit into the hangar of a submarine was one thing. More critical were the design considerations to enable the crew to launch the aircraft within the limited time during which the submarine was surfaced. The expediency of the operation and the critically limited deck space for the crew called for a high level of automation and innovative human engineering.
Ozaki has said the aircraft was originally conceived with no landing gear at all in order to attain maximum speed and range. After the mission the crew would fly back to the sub, ditch the aircraft, then be recovered aboard. Early in the aircraft design study, however, a pair of detachable floats was added as an option, based on a notion that the aircraft could attack several less significant targets before its final one-shot mission. Further, the floats would allow training flights from seaplane ramps and submarines. The submarines were equipped with cranes to recover the airplanes after landing. A provision for jettisoning the floats in flight, although considered during the design process, was never implemented.
Two of the eight prototypes were built with hand-cranked retractable wheeled landing gear. These land-based prototypes were used to establish the aircraft’s characteristics without floats—and once to conduct crew training against a simulated Panama Canal. These two M6A1-K, Shi-Sei Seiran-Kai, were given the name Nanzan (southern mountain) to distinguish them from the seagoing versions. The innovations designed into the Aichi M6A1 made it one of the most advanced and complex Japanese aircraft of the war. The aircraft was probably the one that was least known to foreign military intelligence at that time, as well as to those with technical and historical interests, even today.