I-400-class file photo [6296]

I-400-class Submarine



This article refers to the entire I-400-class; it is not about an individual vessel.

ww2dbaseThe I-400-class submarines of the Japanese Navy were the largest submarines in the world during the WW2-era, and remain so until 1965. They were designed specifically to serve as submarine carriers with the capacity to carry up to three aircraft while still maintaining the typical submarine function as a torpedo launching vessel. The original purpose of these submarines were to carry attack aircraft to destroy the Panama Canal, and an order of 18 boats was issued in 1942. The construction began in Jan 1943 at Kure Navy Yard and Sasebo Navy Yard in Japan. By the end of 1943, the order was reduced to five boats, and ultimately only three would be completed before the end of WW2.

ww2dbaseThe I-400-class submarines employed four diesel engines with enough fuel on board to circumnavigate the world. After Jun 1945, after the installation of breathing tubes, they were able to run their diesel engines while submerged, which greatly extended their speed and range while under water. The three M6A Seiran seaplanes attached to the completed boats were secured on the deck of these submarine carriers, which had special folding systems in the tails and wings that made this carrying capacity possible. There were no equipment on board to recover returning aircraft; the M6A Seiran aircraft were expected to be abandoned at the completion of each mission, with the crews rescued by the I-400-class mother ship or sacrificed.

ww2dbaseThere were notions to use I-400-class submarines to attack Panama Canal or American coastal cities such as New York or Washington DC, but eventually a more realistic target, Ulithi in the Caroline Islands, was chosen. A force of two I-400-class submarines (I-400 and I-401, each carrying three aircraft) and two modified AM-class submarines (each carrying two aircraft) were dispatched. Since the Japanese surrender was announced before the force reached their target, the mission was aborted.

ww2dbaseThe Americans never learned of the existance of the I-400-class submarines until they surrendered. They were studied by the United States Navy at Sasebo, Japan after WW2. Before the Russians got a chance to study these submarines, the US Navy launched Operation Road's End to sink I-402 off Nagasaki, Japan near the Goto Islands. I-400 and I-401 were sent to Hawaii, United States for further inspection; as the Russians again demanded to evaluate these Japanese submarines, I-400 and I-401 were scuttled off Kalaeloa near Oahu, Hawaii on 31 May 1946 to prevent the Russians from doing so.

ww2dbaseSources: Nihon Kaigun, Wikipedia.

Last Major Revision: Aug 2008

I-400-class Submarine Interactive Map

I-400-class Submarine Operational Timeline

13 Jan 1942 Japanese Naval General Staff arranged a meeting with the Bureau of Naval Construction to discuss the construction of submarines capable of carrying attack aircraft.
27 Apr 1942 Japanese Naval General Staff reviewed the preliminary proposal a underwater aircraft carrier.
28 Apr 1942 Japanese Naval General Staff's technology sub-committee reviewed the preliminary proposal of a underwater aircraft carrier.
17 May 1942 Japanese Naval General Staff approved the plans for what was to become the I-400-class submarine design. These would be submarine aircraft carriers conceived to carry out long-range aerial attacks on targets such as the Panama Canal and the continental United States. An order for the construction of 18 examples would be issued within weeks.
29 Sep 1943 The keel of I-403 was laid down by Kawasaki Heavy Industries at Kobe, Japan; the project would soon be canceled, however.
20 Oct 1943 The keel of I-402 was laid down at Sasebo Naval Arsenal, Japan.
30 Dec 1944 I-400 was commissioned into service.
8 Jan 1945 I-401 was commissioned into service at Sasebo, Japan under the command of Lieutenant Commander Nobukiyo Nambu. The ceremony was attended by very few, as she was a top secret submarine. She departed Sasebo later in the same day for the Inland Sea.
11 Apr 1945 I-401 departed Kure, Japan for Dalian, China; she was tasked to bring fuel back to Kure, whose stores were becoming depleted.
12 Apr 1945 En route to Dalian, China, I-401 detonated a magnetic mine at 0850 hours, rupturing the fuel tank and damaging a Kingston valve. She turned around to return to Kure, Japan for repairs.
14 Apr 1945 I-400 departed Kure, Japan for Dalian, China, disguised as a frigate complete with dummy naval guns and machine guns. She was tasked with bringing back fuel from the Dalian storage tanks.
20 Apr 1945 I-400 arrived at Dalian, China and began loading fuel.
1 Jun 1945 I-401 departed Kure, Japan
4 Jun 1945 I-401 arrived at Nanao Bay, Japan
13 Jul 1945 I-400 departed Nanao Bay and arrived at Maizuru, Japan.
13 Jul 1945 I-401 departed Nanao Bay and arrived at Maizuru, Japan.
21 Jul 1945 I-400 departed Maizuru, Japan.
21 Jul 1945 I-401 departed Maizuru, Japan.
22 Jul 1945 I-400 arrived at Mutsu, Japan.
22 Jul 1945 I-401 arrived at Mutsu, Japan.
23 Jul 1945 I-400 departed Mutsu, Japan.
23 Jul 1945 I-401 departed Mutsu, Japan. Shortly after, she was fired upon by friendly Japanese coastal artillery on the island of Hokkaido; she was able to submerge and escaped without damage.
24 Jul 1945 I-402 was commissioned into service.
25 Jul 1945 I-401 began to sail on the surface after traveling submerged for the paste two days.
5 Aug 1945 I-400 developed a electrical power while east of Saipan, Mariana Islands, which quickly developed into an electrical fire that caused the submarine to sink nearly out of control. The repairs took 5 hours to complete. The crew thought that they had surfaced just in time before they ran out of oxygen.
11 Aug 1945 I-402 suffered a punctured fuel tank during an American air attack on Kure, Japan.
13 Aug 1945 I-400 reached the location south of Ulithi, Caroline Islands and waited for I-401 to arrive; her commanding officer did not realize that the rendezvous point had changed to another location 1,000 miles to the west.
14 Aug 1945 In the evening, I-401 arrived at the rendezvous point south of Ulithi, Caroline Islands but did not find I-400; at this time, I-400 was actually at the incorrect location 1,000 miles to the east.
15 Aug 1945 I-401 received conflicting orders, one from Emperor Showa ordering her to surrender in the form of the radio address, another from naval leadership ordering all submarines to carry out existing orders; Tatsunosuke Ariizumi, commanding officer of 6th Fleet who was aboard the submarine, chose to follow the latter. I-400, under Commander Toshio Kusaka, opted to obey the order to surrender.
16 Aug 1945 At 2100 hours, I-401 received specific orders to set sail for Kure, Japan; this order came through just as the submarine was preparing to launch Seiran attack aircraft against the US anchorage at Ulithi, Caroline Islands (Operation Arashi). Tatsunosuke Ariizumi, commanding officer of 6th Fleet who was aboard the submarine and who previously continued the operation despite the country's surrender, now chose to obey. Instead of Kure, however, he set sail for the Sanriku coast in northeastern Honshu island so that the officers and men could disappear into the population rather than having to surrender dishonorably.
20 Aug 1945 I-401 received orders to report in with her current location. Tatsunosuke Ariizumi, commanding officer of 6th Fleet who was aboard the submarine, did not respond.
26 Aug 1945 I-400 dumped all of her ammunition and Seiran aircraft overboard according to orders.
26 Aug 1945 I-401 dumped all of her ammunition and Seiran aircraft overboard according to orders.
27 Aug 1945 I-400 was spotted in the Pacific Ocean by US Navy pilot Lieutenant Robert Mahoney flying a TBM Avenger aircraft. Commander Toshio Kusaka attempted to speed up to escape, but ultimately was ordered by his superiors to cooperate.
28 Aug 1945 Destroyers USS Blue and USS Mansfield intercepted I-400, which was discovered on the previous day, in the Pacific Ocean. USS Blue sent a 37-man boarding party and secured the submarine. Destroyer USS Weaver of US 20th Submarine Squadron arrived on the scene later, however, and forcibly took over the situation, claiming authority due to the personnel's submarine expertise.
31 Aug 1945 Tatsunosuke Ariizumi committed suicide aboard submarine I-401 by taking a pistol to his mouth at 0420 hours. He was buried at sea in a ceremony that somehow evaded the US boarding party.
31 Aug 1945 At 0500 hours, Japanese submarine I-401 lowered its naval ensign while in Sagami Wan, Japan. Nobukiyo Nambu, the submarine's commanding officer, secretly ordered his signal officer to burn the flag to prevent US capture. The US naval ensign was soon hoisted.
30 Sep 1945 I-401's crew was disbanded by the Americans. All officers and men went back into the civilian population, including the few who had committed war crimes.
11 Dec 1945 I-400 departed Sasebo, Japan.
11 Dec 1945 I-401 departed Sasebo, Japan.
19 Dec 1945 I-400 arrived at Guam, Mariana Islands.
19 Dec 1945 I-401 arrived at Guam, Mariana Islands.
6 Jan 1946 I-400 arrived at Pearl Harbor, US Territory of Hawaii.
6 Jan 1946 I-401 arrived at Pearl Harbor, US Territory of Hawaii.
1 Apr 1946 I-402 was scuttled 60 miles off Sasebo, Japan at 1300 hours.
31 May 1946 I-401 was scuttled by two torpedoes from submarine USS Cabezon off Oahu, US Territory of Hawaii; the torpedoes hit her at 1051 hours and she sank at 1058 hours.
4 Jun 1946 I-400 was scuttled by torpedoes from USS Trumpetfish off Oahu, US Territory of Hawaii.

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