Admiral Isoroku Yamamoto’s attack on Pearl Harbor had gone better than anticipated. Although he didn’t destroy any American aircraft carriers, he had succeeded in knocking the United States out of the fight for the Pacific for at least six months. Japan didn’t need to defeat the US outright; she just had to hold off the Americans long enough to secure a footing in Asia.
No one knows for certain when Admiral Yamamoto, commander in chief of Japan’s Combined Fleet, came up with the idea of an offensive squadron of aircraft carrier submarines, but he was toying with the idea at a gathering of his senior officers on Christmas Eve 1941. The admiral needed a way to take the war directly to the US mainland because the psychological effect would be devastating. The way to do it was to build a submarine with the range to reach America’s east coast and return to Japan without refueling. A journey of nearly 38,000 nautical miles.
Although Japanese submarines had been carrying aircraft for nearly two decades, all were seaplanes suited only for reconnaissance. Yamamoto envisioned a submarine capable of carrying three dive bombers. His ultimate plan called for 18 special submarines that could launch a total of 36 attack aircraft—officially referred to as the I-400 class.