Kure Navy Yard, Hiroshima Bay, Japan April 25, 1943

Admiral Isoroku Yamamoto, commander of the Com­bined Fleet of the Impe­rial Japa­nese Navy and the archi­tect of his coun­try’s Decem­ber 7, 1941, attack on the U.S. Pacific Fleet at Pearl Harbor, Hawaii, envi­sioned a dif­fer­ent fleet of Japa­nese air­craft and ships for a second round of attacks on Amer­i­can soil. It would be a bomber fleet of 2‑man float­planes armed with bombs and torpedoes carried aboard and launched from stealthy sub­marines of mam­moth size and oper­a­tional range, off either west or east U.S. coasts. The con­struc­tion of the first of an ini­tially pro­jected 18 under­water air­craft carriers formally began on this date, April 25, 1943, one week and a day after U.S. P‑38 Light­nings shot down the bomber trans­port carrying the admiral to the Pacific Island of Bougain­ville in Oper­a­tion Ven­geance, pay­back for Japan’s deadly attack in 1941.

Production delays and cutbacks in the wake of Yama­moto’s pre­mature death meant that the I‑400 proto­type sub­marine air­craft car­rier was not com­mis­sioned until the end of 1944, nine­teen months later, followed quickly in early Janu­ary 1945 by the I‑401’s com­mis­sioning. Though com­pleted seven months later, in mid‑July 1945, the I‑402 never put to sea (it was finished as a fuel tanker), bringing an end to the mon­ster’s gene­tic line known as Sen­toku, short for Sen-Toku-gata Sensuikan, which loosely trans­lates to “Secret Attack (or Special Type) Sub­marine” in English. Just before the Pacific War ended I‑13 and I‑14, two smaller ver­sions of the I‑400 class, known as Type AM sub­marines (also called I‑13‑class submarines) with the same aspi­ra­tions as the bigger Sen­tokus, made a bow; how­ever, not until nuclear sub­marines appeared in the 1960s did another sub­marine rival the size and operational range of the original I‑400 class.

In January 1945 four underwater aircraft carriers were assigned to the Japa­nese 6th Fleet, 1st Sub­marine Flotilla. Crews learned how to handle the sub­marines and their float­plane bombers. After a year­long intel­li­gence gathering cam­paign, it was decided that the sub­marines’ first com­bat mis­sion would be a sur­prise air strike on the canal locks in the Amer­i­can Panama Canal Zone, through which passed U.S. sol­diers and equip­ment from the East Coast to the Paci­fic Thea­ter. Intel­li­gence esti­mated the two-ocean canal could be rendered unus­able for up to six months were it attacked, and dry runs on a wooden model of the canal’s Gatun Locks gate were conducted in mid-June 1945. The ter­mi­nal phase of the war in the Pacific—the inva­sion of the Japa­nese Home Islands, which the Allies were calling Oper­a­tion Down­fall and which the war­lords in Tokyo reckoned to start in August 1945—prompted a change in plans to instead focus on enemy targets closer to home. Ulithi Atoll, the huge Amer­i­can naval staging area in the Caro­line Islands of the Western Pacific Ocean 1,300 miles/­2,092 m from the Japa­nese capital, was now the target of Japan’s Operation Arashi (Mountain Storm).

The deadly flotilla, comprising I‑400, I‑401, I‑13, and I‑14, departed Japa­nese waters in late July 1945 and slowly proceeded toward Ulithi. Leaving Ulithi’s anchor­age over three months earlier, in March 1945 in the direc­tion of Japan, was a huge con­voy of U.S. Navy ships and person­nel aboard 106 destroyers, 29 air­craft carriers, 15 battle­ships, and 23 cruisers. More war­ships would be departing. On August 16, Arashi flag­ship I‑401 received a radio mes­sage from 6th Fleet head­quarters, informing the crew of their country’s sur­render to the Allies the pre­vious day and ordering the subs to return to base. All six Aichi M6A1 Seiran float­planes on board I‑400 and I‑401 (I‑14 and I‑13 were carrying long-range recon­nais­sance air­craft), having been dis­guised for the oper­a­tion as Amer­i­can planes in vio­lation of the laws of war, were either pushed over­board (in the case of I‑400) or cata­pulted into the sea to pre­vent cap­ture. Japan’s most radi­cal flotilla of the war never dropped a bomb or fired a torpedo in combat.

Japan Builds I-400-Class Submarines to Bring War and Pandemonium to U.S. Mainland

I-400-class submarineAichi M6A1 Seiran seaplane

Left: The mammoth Sentoku, or I‑400-class submarine. Difficult though it is, note the long water-tight, tube-like air­craft hangar on the sub’s aft deck and the forward deck’s com­pressed-air air­plane cata­pult and collaps­ible crane for retrieving returning planes. Japa­nese plans called for building a fleet of 18 I‑400-class sub­marines, at 400 ft/­122 m in length and dis­placing 6,670 tons, by far the largest and among the most deadly subs ever built until the 1960s. (The U.S. Navy’s Balao-class sub­marines, the largest used during the war, were 88 ft/­28 m shorter.) Though Sentokus could fire tor­pe­does (eight on board) like other sub­marines, these super-subs were designed as under­water air­craft carriers, each equipped with three Aichi M6A1 Seiran float­plane bombers. Their mis­sion was to travel more than half­way around the world (the I‑400 had a range of over 30,000 nau­ti­cal miles/­55,560 km and carried a crew of 144 or close to 200 men), sur­face off American coastal cities, and launch deadly aerial attacks. The late-war I‑400-class “wonder wea­pon” was un­known to U.S. intel­li­gence, despite having broken the Japa­nese naval code. All three I‑400-class subs were scuttled after having been studied by U.S. naval engi­neers: I‑400 and I‑401 were torpe­doed as target ships near Oahu, Hawaii, on June 4 and May  31, 1946, respec­tively. I‑402 was scuttled off Nagasaki, Japan, on April 1, 1946.

Right: A two-man Aichi M6A1 Seiran floatplane, the type carried aboard the I‑400– and smaller I‑13‑class subs. The super sub­marines were designed to carry two (in the case of I‑13‑class subs) or (three I‑400-class) Seirans, a name that trans­lates as “mist on a clear day” or “clear sky storm.” Each Seiran float­plane was cap­able of carrying one 1,800‑lb/­816 kg tor­pedo or the equi­va­lent weight in bombs for up to 739 miles/­1,189 km. A well-trained crew of four men could roll a Seiran out of its water­tight hangar on a col­laps­ible cata­pult car­riage, unfold the tail and wings pressed against the fuse­lage, and have it readied for flight in approx­i­mately 14½ minutes or twice that time if the plane’s pon­toons were fitted. Twenty-eight Seirans, which included 8 proto­types used in testing, were com­pleted by its manu­fac­turer, Aichi, before pro­duc­tion was halted due to the des­per­ate shrink­age in the late-war size of the carrier sub­marine force. After a resto­ra­tion that took 11 years, a soli­tary survi­ving M6A1 can be viewed in the Udvar-Hazy Center of the Smith­son­ian National Air and Space Museum located in Chan­tilly, Virginia, a suburb of Washington, D.C.

U.S. Navy personnel inspect I-14, Novem­ber 1945(L-R) USS Proteus, I-14, and I-400 aircraft carrier submarine

Left: American naval personnel inspect I‑14, Novem­ber 1945. Likely photo­graphed near Sasebo Naval Arsenal, Japan, where she joined the com­pany of I‑400 and I‑401. The Type AM (or I‑13-class) sub­marine was orig­i­nally designed as a com­mand sub­marine carrying recon­nais­sance float­planes, but it was capa­ble of carrying two Seirans. The large struc­ture beneath the offset con­ning tower, clearly distin­guish­able in this photo, is the 11‑ft/­3.35‑m-wide cylin­drical hangar that could hold two planes with folded wings and tail fin. The I‑14 was armed with six internal bow tor­pedo tubes and carried a total of a dozen torpe­does. The sub was also armed with a single 140mm/­5.5‑in deck gun and one single- and two triple-mount anti‑aircraft guns. The I‑14 carried enough fuel to sail around the globe one and a half times. Her sister ship, I‑13, was sunk on July 16, 1945, by a U.S. destroyer escort and carrier air­craft some 633 miles/­1,018 km east of the main Japa­nese island of Honshū. Flying the black flag of surren­der, I‑14 surren­dered at sea on August 27, 1945, 227 miles/­365 km miles north­east of Tokyo, and was scuttled in tar­get prac­tice off the Hawaiian island of Oahu on May 28, 1946.

Right: Sailors line the rails of the submarine tender USS Proteus, while other men crowd the forward deck of I‑14 (center in photo), a smaller ver­sion of I‑400 seen at the right. Photo­graphed in Tokyo Bay at Yoko­suka Naval Base in the lagoon adja­cent to the Japa­nese sub­marine base, Septem­ber or Octo­ber 1945. The large, bullet-shaped struc­tures on the forward decks of both sub­marines are the outer access doors to the air­craft water­tight hangars. Clearly seen on I‑400 is the 85 ft/­26 m com­pressed-air cata­pult mounted on the for­ward deck that launched the sub’s three Aichi M6A Seiran attack floatplanes.

Japan Builds Secret Weapon: Aircraft-Carrying I 400–Class Super Submarine