In 1942, while the Manhattan Project was rolling along in the United States, the Japanese were plotting their own secret weapon: a submarine aircraft carrier. Carrying a plane on a submarine was not a new idea, but those were just scout planes. The I-400 was designed to carry three bomber planes.
Admiral Yamamoto conceived the idea of the Japanese Super Sub. He had specific requirements for the submarine, and every one of them was met. In the end, the submarine could travel around the entire world one-and-a-half times without refueling; it could carry an aircraft hangar big enough for three planes; it contained eight torpedo tubes and numerous anti-aircraft weapons; and, with the help of the Germans, it could easily evade sonar devices as well.
With all its new gadgets, the I-400 needed a new design. Because of the weight of the hangar and planes, the I-400 would have been too top heavy to float properly. Instead of making the entire hull larger, creating thicker walls, and therefore a much heavier submarine, the Japanese created a double hull design. The I-400 looked similar to two submarines welded together to create a securer base for the heavy hangar.
These planes inside the I-400 were also specific to the submarine, featuring folding wings and tail, and detached undercarriage. Besides fitting in the hangar, the Japanese decreased the Seiran aircraft’s takeoff time by funneling preheated oil into their engines. Instead of taking a quarter of an hour or more for the engine to warm up, the planes would be able to take off in a few minutes while the I-400 quickly dove back down. To get the planes back onto the deck, the submarine featured a hydraulic crane that could lift the planes from the water.
When all the design challenges had been figured out, Admiral Yamamoto ordered 18 of the submarines to be built. However, only three were finished. Admiral Yamamoto was killed on a tour to the Solomon Islands in 1943 by the United States Navy, and his project fell to the wayside.
As the war continued, Japan could not keep up with the firepower of the Germans and the United States, resulting in other warfare tactics such as kamikaze missions. The Sairen itself could only hold one bomb and was therefore not very useful in terrorizing major cities as the Germans did in London with blitzkrieg. Instead of bombing heavily populated areas, the Japanese decided to focus on a target crucial to the United States: the Panama Canal. But before they could get their chance, Allied forces attacked the island of Okinawa. The Japanese Navy then changed their target from the Panama Canal to Ulithi atoll, where many Allied ships were preparing to invade Japan.
Four days before the submarines’s launch to Ulithi, the United States successfully tested the first atomic bomb, and the second one was in flight to Japan. The crews heard the news of the bombings of Hiroshima and then Nagasaki. It wasn’t long before the Japanese surrendered and the I-400 crews were ordered to destroy their weapons (including the planes) and return to Japan, but the men were captured before returning home.
The massive and intriguing design of the submarine surprised the Americans, who had never seen anything like it. After inspection, both the Americans and Japanese worked together to get the crew and submarines back to Tokyo. Once the Japanese returned to their land, the United States took the submarines back to America for further study. Naval engineers quickly inspected the new machine, but it wasn’t long before someone else was curious. The Soviet Union heard word of the large sea monsters. Before the Soviets could demand inspection, the United States sank the submarines; the I-400s never had the chance to battle.
Read about the capture of the I-400 in the Submarine Patrol Reports when the “large Japanese Submarine” was first identified, and learn more about the Super Sub and the Japanese’s strategies to attack America on PBS from Secrets of the Dead.