In August of 1945, as the world celebrates the end of the Second World War, out in the pacific the Americans make a puzzling discovery. The US Navy has intercepted a Japanese submarine and it is unlike anything they have seen before. Its scale is baffling. However, not only is it the world’s largest submarine, it is an entirely new kind of weapon. This submarine can launch torpedo dive-bombers. The American’s have just stumbled across Japan’s secret underwater aircraft carriers, and soon they will uncover a sinister plan that could have changed the course of the war. Japan’s surprise attack on Pearl Harbor in December 1941 is an event that galvanizes a nation. Pulling a reluctant America into the Second World War. “The unprovoked and dastardly attack by Japan on Sunday…” A day after the devastating attack, the United States declares war on Japan. The nation quickly mobilizes, firing up its enormous industrial might to crank out ships, tanks and aircraft at a rate that will soon bury Japan’s military.
For the Americans, Pearl Harbor was a senseless and cowardly provocation. However, for the Japanese, the attack was something entirely different. A calculated gamble and a long shot attempt at actually trying to avoid a full-scale war with the United States. Because the Empire of Japan continued its ruthless conquest in Asia, the Japanese were convinced that it was only a matter of time before the US intervened. The architect behind the Pearl Harbor attack, Japanese Admiral Yamamoto was aiming to knock out much of the US Pacific fleet in one decisive blow. As a way to keep the United States out of the Pacific for at least another six months and perhaps even forcing the Americans into negotiating a truce. However, Imperial Japan grossly underestimated America’s resolve. In the aftermath, Yamamoto knew America’s military might will overwhelm Japan’s military power. Now faced with a war he cannot win, Yamamoto devises another strategy. To make America reconsider a drawn out war in the Pacific, He will bring the war directly to America’s Cities. However, with the United States now on guard for Japanese forces, Yamamato will need a truly stealthy weapon to reach the United States. A weapon the Americans would never suspect. The concept of launching aircraft from a submarine originated before the Second World War. However, these earlier attempts were experimental trials usually involving a single lightweight reconnaissance plane. What Yamamoto had in mind was far more ambitious. A fleet of submarines that could carry multiple attack aircraft and strike fear into the enemy by launching surprise attacks on cities, only to submerge and disappear again
In March of 1942, the military leadership handed Japanese engineers the enormous task of designing Yamamoto’s secret weapon. To start, Yamamoto’s aircraft carrying subs would need to be capable of launching full-size torpedo dive-bombers. Engineers would have to design a catapult launching system. They had design a mechanism to recover the aircraft and bring them back onboard. However, making the bombers fit inside a submarine would be an even bigger challenge. With a typical World War Two-era dive-bomber having a wingspan of about 12 meters, engineers would also need to design an entirely new dive-bomber. One that engineers could fold up to fit inside the sub’s hanger. The submarines would also need to be able to reach any part of America’s coastline, thousands of kilometers away. Then return all the way back to Japan without refueling or resupplying. It meant carrying nearly two million liters of diesel fuel and enough supplies to support a crew out at sea for months. The military leadership designated Japan’s secret underwater aircraft carriers as the I-400. They would be enormous. They were nearly twice the length of a typical German U-boat. To support the weight of the hanger and to keep the sub stable during carrier operations, engineers created a double hull design. Essentially two hulls stuck together. It gave the mammoth I-400 nearly three times the displacement of even the largest American submarine. The I-400 was still a formidable submarine in the conventional sense. Engineers armed it with eight forward mounted torpedo tubes and a massive deck gun. To fend off enemy aircraft, three triple-mounted anti-aircraft guns and a fourth single mounted gun on the sail. However, of course the I-400’s primary weapons were its three torpedo dive-bombers. The element of surprise was an underwater aircraft carrier’s greatest advantage. As the I-400 silently approached its target, its crew would already begin preparing the aircraft. Mechanics would start by running heated oil through the aircraft’s engines so they would be warmed up and ready to launch. The mammoth submarine would surface a few hundred kilometers from its target and the race would be on to get three bombers airborne. Each aircraft would be rolled out from the hangar onto the deck. Crews would then start the engine, unfold the wings and tail, lock floats into place, and load armament. One by one, the three aircraft would be launched using a compressed air catapult. The whole process would take thirty minutes. After which the I-400 would dive back to safety and silently wait for the bombers to return from their mission. The torpedo dive-bombers were cutting edge. They were designed specifically for the I-400 and could carry the largest bomb or torpedo in Japan’s naval arsenal. Equipped with floats, the aircraft would land alongside the submarine to be hoisted back aboard using a collapsible hydraulic crane. The aircraft could also be launched without floats for greater range and performance, but forcing pilots to ditch into the ocean after their mission. The I-400 was a brilliant design, merging the stealth of a submarine with the offensive strike capability of an aircraft carrier. However, Japan’s new super weapon would make no difference in the War.
On August 15, 1945, after nearly four years of brutal conflict and with American forces closing in and the bombing of Japanese cities, Japan finally surrendered. The Americans first intercepted an I-400 off the coast of Japan two weeks after the surrender. At first they were not exactly sure what it actually was. The Japanese crew had thrown all of the attack aircraft overboard. At first the Americans believed the I-400 was designed to carry cargo. However, they would soon unravel the submarine’s true purpose and why Japan never used them in the War. To start, many in Japan’s Navy considered Yamamoto’s underwater aircraft carriers to be a farce. The slow process of launching aircraft in the middle of a combat zone too dangerous for submarine crews. However, resistance to the concept would soon be the least of Yamamoto’s concerns. Because it took nearly a year to design such an unconventional weapon. Construction of the first I-400s only began at the start of 1943. By then, the Japanese were already losing the War. After a crushing defeat in June of 1942, the Americans were pushing the Japanese back across the pacific. Japan was running critically short on fuel and raw materials. Delaying I-400 construction even further. Yamamato himself would never live to see any of his submarines completed. In 1943, while on an inspection tour through the South Pacific, Yamamoto’s plane was downed by American forces. What started as a plan to build a fleet of eighteen underwater aircraft carriers was eventually whittled down to just five. Only three were ever completed. The first entering service in 1945. So late in the War that Japan’s military had already all but collapsed. Launching sneak attacks on American cities with a handful of dive-bombers would have been pointless. Even a strategic mission to bomb the Panama Canal was abandoned after Japanese command felt that it too would have made little difference so late in the War.
The only mission the I-400’s would ever set out on was a last ditch effort to bomb American forces as they amassed off a tiny pacific atoll. However, as the first I-400s traveled to their targets, Japan surrendered, finally ending the Second World War. Aircraft carrier submarines had always been a gamble. A way to change odds so stacked against Japan, that only through sheer ingenuity could the tables be turned. Had the sub arrived at the start of the war it might have made a difference. However, Japan’s secret weapon was not without compromise. The process of launching three aircraft was supposed to take 30 minutes. However, rarely could it be accomplished in less than 45 minutes. A dangerous amount of time for such a large submarine to be surfaced. The I-400’s bombers, while sophisticated in their design were rushed into service and built from lower grade materials due to shortages. They were notoriously unreliable. Rarely could all three get airborne without some mechanical problem. The enormous I-400’s depth time, critical for getting out of danger, was
PRIMARY SOURCES: Mustard Videos on Japanese Submarines