In World War II, This Unusual Airplane Could Launch from a Submarine

The Smithsonian has the world's only surviving example of a unique Japanese bomber.

During World War II, Toshio Ozaki—chief engineer at Japan’s Aichi Kokuki aircraft manufacturing company—designed the Aichi M6A1 Seiran (“clear sky storm”) to fulfill the requirement for a bomber that could operate exclusively from a submarine. Japanese strategists hoped to use the bomber to strike the U.S. mainland and other important far-away targets, such as the Panama Canal. No Seiran ever saw combat, but the design represents an ingenious blend of aviation and marine technology.

The National Air and Space Museum has the world’s only surviving example of the M6A1. Kazuo Akatsuka, a lieutenant in the Imperial Japanese Navy, ferried it from Fukuyama to Yokosuka, where he surrendered it to a U.S. occupation contingent. The aircraft was then periodically displayed at Naval Air Station Alameda in California until the U.S. Navy transferred it to the Smithsonian in 1962. Restoration work began on the M6A1 in June 1989 and ended in February 2000. The M6A1 is on display at the Museum’s Steven F. Udvar-Hazy Center in Chantilly, Virginia.

To fit inside an 11.5-foot-diameter submarine hangar, the aircraft was designed so that the tip of the vertical stabilizer and about two-thirds of each side of the horizontal stabilizer could be folded downward.

The wing’s main spar was designed to turn 90 degrees. After rotating the wings, the deck crew folded them back to lie flat against the fuselage for storage.

The Seiran's cockpit has a tandem seating arrangement for the pilot and an observer . The observer, who served as the navigator and radio operator, could also swivel backward to fire a Type 2, belt-fed 13mm machine gun. 

The compact design of the Aichi Atsuta 31, Ha 60 Model 31 liquid-cooled engine enabled designers to reduce the size of the aircraft’s nose, decreasing drag and increasing aerodynamic efficiency.

If deck crews didn’t attach the floats, they could launch three aircraft in less than 15 minutes. The lack of floats, however, meant pilots returning from a bombing mission would have to ditch near a submarine and await rescue. 

This article, originally titled "Storm in a Clear Sky," is from the Summer 2025 issue of Air & Space Quarterly, the National Air and Space Museum's signature magazine that explores topics in aviation and space, from the earliest moments of flight to today. Explore the full issue.

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