Contributor: C. Peter Chen
This article refers to the entire I-400-class; it is not about an individual vessel.
The I-400-class submarines of the Japanese Navy were the largest submarines in the world during the WW2-era, and remain so until 1965. They were designed specifically to serve as submarine carriers with the capacity to carry up to three aircraft while still maintaining the typical submarine function as a torpedo launching vessel. The original purpose of these submarines were to carry attack aircraft to destroy the Panama Canal, and an order of 18 boats was issued in 1942. The construction began in Jan 1943 at Kure Navy Yard and Sasebo Navy Yard in Japan. By the end of 1943, the order was reduced to five boats, and ultimately only three would be completed before the end of WW2.
The I-400-class submarines employed four diesel engines with enough fuel on board to circumnavigate the world. After Jun 1945, after the installation of breathing tubes, they were able to run their diesel engines while submerged, which greatly extended their speed and range while under water. The three M6A Seiran seaplanes attached to the completed boats were secured on the deck of these submarine carriers, which had special folding systems in the tails and wings that made this carrying capacity possible. There were no equipment on board to recover returning aircraft; the M6A Seiran aircraft were expected to be abandoned at the completion of each mission, with the crews rescued by the I-400-class mother ship or sacrificed.
There were notions to use I-400-class submarines to attack Panama Canal or American coastal cities such as New York or Washington DC, but eventually a more realistic target, Ulithi in the Caroline Islands, was chosen. 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) were dispatched. Since the Japanese surrender was announced before the force reached their target, the mission was aborted.
The Americans never learned of the existance of the I-400-class submarines until they surrendered. They were studied by the United States Navy at Sasebo, Japan after WW2. Before the Russians got a chance to study these submarines, the US Navy launched Operation Road's End to sink I-402 off Nagasaki, Japan near the Goto Islands. I-400 and I-401 were sent to Hawaii, United States for further inspection; as the Russians again demanded to evaluate these Japanese submarines, I-400 and I-401 were scuttled off Kalaeloa near Oahu, Hawaii on 31 May 1946 to prevent the Russians from doing so.
Sources: Nihon Kaigun, Wikipedia.
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General Douglas MacArthur at Leyte, 17 Oct 1944