The Imperial Japanese Navy, during the Second World War, succeeded in building a model of submarine capable of transporting aircraft that, until the entry into service, in the 1960, of nuclear-powered submarines, remained the largest boats ever built, with a displacement equal to 6.600 tons.
From a strategic point of view, the I-400 class can be considered as a predecessor of the current SSBNs.
Called I-400 they were designed to go around the globe without refueling and conduct attacks directly on American territory.
Initially the program - set in the 1942 with the support of Admiral Yamamoto - included the creation of exemplary 18, while construction work began in 1943 and the first boat was built in Kure's naval arsenals in Hiroshima. However the delays accumulated following the death of Yamamoto, which took place on the 18 1943 in April, allowed the construction of only three boats (I-400 in Kure, I-401 and I-402 in Sasebo)
Three Aichi M6A aircraft could be loaded Seiran, specifically designed to take off from class I-400 submarines, had an operating radius of 1.500 km at a speed of 555 km / h, and were able to carry an anti-aircraft anti-towable torpedo or the equivalent in weight of bombs for a maximum of 850 kg.
Furthermore, the I-400 class was armed with 8 mm torpedo launch tubes; a naval cannon of 533 mm; three towers with 140 Type 96 mm pieces and a single piece of the same model. The boats could reach the surface speed of 25 knots, while in diving it descended to 18.
Yamamoto's strategy was to succeed - by means of the I-400 class - in carrying out attacks along the western coast of the United States, in such a way as to divert large quantities of men and means, to protect American territory, from operations against possessions Japanese in Asia.
The I-400 class boats were unwieldy and difficult to maneuver during emergence due to rudders that were too small. Moreover, given the excessively large superstructure, a strong wind could take them off course.
Despite design flaws, the Imperial Navy, given the turn of the war against Japan, devised a bold plan to attack the Panama Canal, with the aim of blocking American supplies from the Atlantic Ocean.
The reconnaissance operations began in August of the 1943, the idea was to destroy the locks of the Canal, thus blocking the flow of ships to the Pacific. Obviously there were extensive American fortifications along both sides of the Canal, to protect it from possible enemy attacks.
The Japanese plan foresaw the three class I-400 boats would stop at around 185 km off the coast of Ecuador, and let the Seiran no floats, flying at an altitude of 4.000 meters along the northern coast of Colombia.
With the increasingly disastrous course of the war for Japanese weapons, the operation was more and more procrastinated, meanwhile we thought of kamikaze rather than conventional bombardment.
Despite the exercises going ahead for the operation against the Canal, the American landing in Okinawa and the subsequent fall of the island forced the Imperial Navy to modify its plans against the Canal to defend the metropolitan territory, in view of the increasingly probable invasion ally.
An American naval group composed of 15 aircraft carriers had gathered in Ulithi's atoll, in preparation for the landing in the Japanese Archipelago, so the I-400 class submarines received orders to attack them.
The attack plan foresaw that the I-400 and I-401 boats, after meeting one set point, would launch their six Seiran the night of the 17 August 1945 against the enemy naval formation. Each plane would carry an 850 kg bomb and, after a flight at an altitude of 50 meters above sea level to escape the radars, they would have had to crash over the carriers' decks (and the Japanese aircraft were painted in American colors to deceive the enemy).
Japan surrendered before the attack was launched, submarine crews were ordered to destroy all weapons on board. The US Navy recovered all three I-400 class boats and took them to Sasebo Bay to study them.