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Lieutenant Commander Stephen L. Johnson faced a massive Japanese submarine, the I-401, off the coast of Honshu, Japan. The submarine was part of the I-400 squadron, which consisted of underwater aircraft carriers. Commander Tatsunosuke Ariizumi, the pilot of the I-401, was responsible for creating the top-secret submarines planned to launch surprise attacks against the United States mainland. Lt. Cmdr. Nobukiyo Nambu, the skipper of the I-401, was in charge of the largest submarine in the world, built to accommodate three cutting-edge attack planes. After the attack on Pearl Harbor, Admiral Isoroku Yamamoto, the commander of Japan's Combined Fleet, ordered the I-400 series to be built. Yamamoto proposed a submarine that could cover 40,000 nautical miles without refueling, carry two attack planes, and move 6,560 tons underwater.
Yamamoto ordered the construction of 18 Sen-toku submarines, capable of holding thirty-six attack planes. The submarines were designed to be redesigned for speed, range, and bomb payload size. The Aichi Aircraft Company was contracted to handle the plane's design, testing, and construction. The Japanese navy decided to attack the Panama Canal to slow down the American war machine in the Pacific. The Aichi attack plane, Seiran, conducted its first test flight in November 1943. The Japanese launched the offensive in early 1945, but faced issues due to a shortage of steel in the navy. Despite these challenges, the Japanese completed the I-400 and smaller I-13 submarines, and the I-14 was operational by mid-March 1945. Vice Admiral Jisaburo Ozawa proposed using the Sen-toku submarines with biological weapons against a populated region on the US West Coast, but the Army rejected the idea due to the perception that the war was already lost.
In 1945, the Japanese Navy had only twenty contemporary submarines, including those of the Sen-toku squadron. The I-400 subs were tested to test-launch their Sieran planes, but issues arose, including operational issues, fuel shortages, and mines. The plan was to launch the planes in June, but the I-14 proved challenging to train on. The Americans' anxiety increased with the loss of Iwo Jima and the American onslaught on Okinawa. The planned attack on the Panama Canal was canceled, and the I-14 and I-13 were turned over to Japan. The submarines returned to Tokyo Bay, attracted attention, and the U.S. Navy purposefully sank all three by early June 1946 to keep them hidden from the Soviets. After the war, one of the Seirans was restored and is now housed at the Smithsonian Air and Space Museum. The I-400s served as the strategic forerunners of modern ballistic submarines, particularly the Regulus missile program.
