Designed in 1942, the I-400-class submarine was to carry three to four light Aichi Type 17 Special Fighter aircraft (M6A1) to strike the U.S. mainland. The idea of a "submersible aircraft carrier" was unique to the Japanese navy of World War II. The I-400-class submarines were powered with four diesel engines, generating 7,700 hp. Their maximum speed was 18.7 knots on the surface and 6.5 knots submerged, with a cruising radius of 42,000 nautical miles at 14 knots. A ship of the I-400-class was thus capable of steaming around the world without refueling. The I-400s carried a standard armament of one 144 mm gun and three 25 mm machine guns. It took 30 minutes to catapult an airplane from the submarine. Each aircraft could carry two 550 lb bombs or one 1,760 lb bomb. This type of submarine and the Japanese balloon bombs were the only real means available for Japan's military to attack the mainland United States during the Pacific war.
The development concept behind the I-400 was similar to that of the superbattleship Yamato, in which the Japanese navy pursued the goal of having the largest guns and ships. However, the I-400 could not be mass-produced. In 1942, the Japanese navy planned to build 18 I-400s, but only 3 were completed during the war. Nor could the Japanese navy use them effectively, given Allied control of the Pacific. In 1945, the navy developed a plan to utilize the I-400s in an attack on the Panama Canal. The attack was scheduled for 25 August 1945 but was canceled with the Japanese surrender. After the war, two I-400 submarines were handed over to the U.S. Navy. The remaining I-400 was broken up for scrap by the Japanese in 1946.
Boyd, Carl, and Yoshida Akihiko. The Japanese Submarine Force and World War II. Annapolis, MD: Naval Institute Press, 2002.; Fukui Shizuo. Nihon Sensuikan Monogatari (The story of Japanese submarines). Tokyo: Koujinsha, 1994.; Walts, Anthony, and Brian Gordon. The Imperial Japanese Navy. London: Macdonald, 1971.