The I-400s were specifically designed as underwater aircraft carriers to support the M6A1 Seiran, designed by Aichi’s chief engineer, Toshio Ozaki, and built in the company’s Nagoya factory. The Seiran was intended to strike directly at the U.S. mainland. Unlike previous sub-based aircraft designed for reconnaissance or defensive measures, it was a purely offensive weapon built to command respect.

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In the book I-400: Japan’s Secret Aircraft-Carrying Strike Submarine, Lieutenant Ta­dashi Funada, a test pilot who flew the first Seiran prototype, is credited with naming the aircraft. The name Seiran is composed of two Japanese words that can be translated as “storm out of a clear sky.” According to the authors, Lieutenant Funada’s hope was that the bomber would gain the key element of surprise by suddenly seeming to appear out of nowhere.

Aichi completed the first Seiran prototype in the fall of 1943, and the Imperial Japanese Navy was happy enough with the result to order production to start immediately. The original production goal of 44 aircraft was eventually reduced to 28 (including two M6A1-K trainers) due to the plane’s cost and war-driven material shortages, not to mention two major earthquakes and relentless bombing by B-29s, both of which damaged Aichi’s Seiran factory.

Former Lieutenant Atsushi Asamura, the leader of Squadron Number 1, which was re­sponsible for the planned attack on the Panama Canal, confirmed the difficulties surrounding Seiran production. Interviewed in his Tokyo apartment, the 86-year-old former pilot said, “The Seirans that were custom built were of good quality, but as they scaled back production the quality became poor due to material shortages and difficult manufacturing conditions.” In fact, many of the Aichi employees responsible for building the Seirans were high school students.

Nevertheless, Lieutenant Asamura, who remains fit and speaks in a strong voice, recalled the Seiran as “a good performance aircraft,” confirming its reputation as streamlined and responsive, with excellent attack power. “It was a versatile plane since it was both an attack bomber and had long distance range,” Asamura said, illustrating the Seiran’s easy handling by holding his arms out like wings, then grabbing an imaginary stick. “But there was no big difference in how it handled a sea landing compared to other planes.”

Asamura also recalled that the Seiran’s liquid-cooled engine provided pilots with much better visibility than the bulkier and more common air-cooled engines in use at the time. The Atsuta 30 series 12-cylinder inverted Vee engine (Japan’s version of a German Daimler-Benz DB 601A) delivered 1,400 hp, and its liquid-cooled design meant it didn’t need as much warm-up time as an air-cooled engine, so the plane could launch faster. Given the danger subs faced on the surface, this was a distinct advantage.

The Seiran featured a metal frame construction with a riveted metal fuselage and triple-blade propeller. It required a crew of two: a pilot and an observer who sat in a tandem configu­ration. The observer served as radio operator and navigator, also manning the flexible rear-facing 13mm machine gun, which flipped up from a recess in the fuselage and locked into place for firing. The aircraft carried either a 551-pound bomb with its floats attached or a 1,764-pound bomb (or torpedo) without floats. The heavier ordnance meant that the pilot would have to ditch the plane upon his return, or it was a one-way suicide mission.

By necessity, the Seiran had hydraulically folding wings similar to the Grumman F6F Hellcat’s that rotated 90 degrees to ensure the aircraft fit inside its small, tubelike hangar, which was only 11 feet 6 inches in diameter. Part of the horizontal stabilizer and the tip of the vertical stabilizer also folded down to accommodate the tight fit. The plane’s floats were detachable and stored separately, as were their support pylons and spare parts.

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