World War II Database

Submarine I-400 was the lead craft of her class of underwater aircraft carriers, capable of carrying up to three M6A Seiran seaplanes. Built at Kure, Japan, she (later joined by her sister submarines) became the largest submarine in the world, and would remain so until 1965. On 15 Dec 1944, Commander Toshio Kusaka was assigned as her commanding officer. On 30 Dec 1944, she was assigned to Submarine Division 1 of the Sixth Fleet. On 19 Mar 1945, she was strafed by American aircraft while in drydock at Kure, killing one anti-aircraft gunner. On 12 Apr, she departed Kure for Dairen, Manchuria, China, but was lightly damaged by a naval mine and had to return to Kure for repairs; she made the trip again on 14 Apr and made it to her destination successfully. She returned to Kure on 27 Apr. In May 1945, she was fitted with a breathing tube to allow her diesel engines to operate while she was submerged. In early Jun 1945, she conducted an exercise in Nanao Bay, launching six M6A Seiran aircraft operated by the 631st Naval Air Group.

I-400 was originally designed to attack faraway targets such as American coastal cities or the Panama Canal, but toward the end of the war when she was ready to launch an attack, a more realistic target was chosen. On 20 Jul 1945, she was dispatched from Maizuru harbor during Operation Arashi in a force of two I-400-class submarines (I-400 and I-401, each carrying three aircraft) and two modified AM-class submarines (each carrying two aircraft) to attack the American anchorage at Ulithi in the Caroline Islands. On 22 Jul, she arrived at Ominato, Honshu, Japan, where all crew were granted one day's leave while all submarines in the group were painted with fake American markings. On the next day, she departed at 1400, taking a slightly different route toward Ulithi than the other submarines to avoid detection. Since the Japanese surrender was announced before the force reached their target, the mission was aborted on 18 Aug. All three I-400-class submarines fired their torpedoes and launched their aircraft into the water on 22 Aug 1945 before surrendering to the American destroyers Blue and Mansfield 500 miles northeast of Tokyo, Japan at 1200 on 27 Aug. At 0500 on 31 Aug, the American flag was raised above I-400 for the first time.

The Americans never learned of the existence of the I-400-class submarines until their crews surrendered. I-400 and I-401 were studied by the United States Navy at Sasebo, Japan between 1 Nov and 11 Dec 1945 and then at Hawaii, United States between 18 Feb and May 1946. As the Russians increased their demands to evaluate the Japanese submarines as well, I-400 was sunk as a target ship off Kalaeloa near Oahu, Hawaii on 4 Jun 1946 to prevent the Russians from doing so. She was sunk by three torpedoes launched by submarine USS Trumpetfish.

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