Design work on this new vessel class in 1942 with construction formally beginning in early 1943. The shipyards at Kure, Hiroshima shipyard and Sasebo was tabbed with manufacture of the large submarines. Originally, it was envisioned that 18 such vessels would be completed. However, this order dwindled to half that amidst mounting IJN losses (Yamamoto himself was killed during a trip to the Solomon Islands), shortages of wartime supplies and waning interest in the project itself. The nine-strong order then became five and, of these, only three ever reached a completed, usable form. These three were designated the I-400 (the lead ship of the class) and sisters I-401 and I-402.
The I-400 class was forged into a mammoth submarine designed for both the rigors of open ocean sailing and diving to depths of 328 feet. As such, the vessel was highly pressurized including the integrated hangar needed to house the floatplane aircraft. While original plans called for two such aircraft to be housed, the I-400 class was designed to fit three - these being the Aichi M6A1 "Seiran" series of floatplane aircraft. Outwardly, the submarine was rather conventional in her appearance with a smoothly contoured shape from bow to stern. It took on a traditional cylindrical form save for the top facing which was kept traditionally flat t serve as a suitable platform for when the vessel was surfaced. The propeller systems were held at the extreme rear section of the craft and driven by four diesel-fueled engines when surfaced and two powerful electric motors when submerged. The engines were tied to a gearbox which powered a pair of propeller shafts. A rudder was affixed at the extreme end of the boat for turning.
The design yielded a running length of nearly 400 feet with a beam wide enough for the required aircraft at just over 39 feet. The I-400 was given a draught 23 feet deep and yielded a displacement of 6,500 short tons. The pressurized hanger was fitted amidships which forced the bridge superstructure to be offset to the portside from centerline. The hangar was accessible from within the vessel itself or from outside along the deck. The foredeck was dominated by the catapult launching system running from bridge superstructure to bow and slightly angled upwards to promote short take-offs for the floatplanes. A collapsible crane arm was also fitted ahead of the bridge and to be used in recovering the floatplane aircraft as needed.
The I-400 was defensed by 1 x 140mm (5.5") deck gun and 4 x 25mm Type 96 anti-aircraft cannons. The deck gun was fitted aft of the bridge superstructure while a pair of the 25mm triple-mounted cannons were fitted aft of the tower proper, along the superstructure platform. The remaining 25mm triple-mounted cannon was mounted forward of the tower along the same platform. A single-mounted 25mm cannon was fitted to a pedestal mount aft of the bridge. This network was intended to fight off any low-flying aircraft. The floatplane aircraft allotted their own supply of ordnance in the form of conventional drop bombs and torpedoes. The I-400 maintained her attack submarine nature as well, being fitted with 8 x 533mm torpedo tubes, all facing towards the bow, with 20 torpedo reloads intended to counter Allied surface vessels.
Her uniqueness in design also required a different approach to her hull construction. Instead of a basic cylindrical cross section form, the I-400 class was completed with a horizontally-set "figure eight" cross section (when viewed in its forward profile) required of her broad structure and to withstand the pressure of her expected operating depths. The vessel also made use of typical submarine installations such as day and night periscopes (of German origin), radar warning receivers and aircraft / surface ship detecting radar facilities.
Performance specifications for the I-400 class included a top surfaced speed of 19 knots with a submerged speed equal to 7 knots. Japanese submarines always lacked in the latter category for they never matched the specifications exhibited of her contemporaries elsewhere. Surfaced range was an impressive 4,350 miles though this was substantially lessened when submerged to just 68 miles when on battery power. As with other diesel-electric submarines of the time, the I-400 needed to surface to recharge her batteries and oxygen supply - making this the most vulnerable time for the craft and crew. A fixed snorkel allowed the vessel to stay just under the waterline at periscope depth for stealth-minded coastal work while at the same time recharging the battery stores.
Despite the ambitiousness in her design, the I-400 class never amounted to much for the of the war soon found the Japanese Empire. The three completed vessels never saw operational service though two were commissioned.
I-400 was constructed by the Kure Naval Arsenal and laid down on January 18th, 1943, launched on January 18th, 1944 and officially completed on December 30th, 1944. She was subsequently captured by USS Blue on August 19th, 1945.
I-401 was constructed by the Sasebo Naval Arsenal and laid down on April 26th, 1943, launched on March 11th, 1944 and completed on January 8th, 1945. She was subsequently captured by USS Segundo on August 29th, 1945.
I-402 was constructed by the Sasebo Naval Arsenal and laid down on October 20th, 1943, launched on September 5th, 1944 and completed on July 24th, 1945. She was not officially commissioned and ultimately converted from submarine aircraft carrier to a submarine tanker resupply vessel. She was formally decommissioned on November 15th, 1945.
I-400 and I-401 were transferred by the Americans to the Hawaiian islands for evaluation and eventually sunk off of the coast of Oahu - the I-400 on June 4th, 1946 and the I-401 before her on May 31st, 1946. I-402 was sunk off the coast of the Goto Islands near Japan on April 1st, 1946. All this was done after Allied inspections of her technology were completed and in an effort to avoid allowing Soviet engineers their own first-person evaluations.
I-403, I-406, I-407, I-408 and I-409 were all cancelled in October of 1943. I-404 was some 95% of the way completed when an Allied bombing raid damaged her beyond repair. She was subsequently scuttled and scrapped - post-war - in 1952. I-405 began construction but the endeavor was halted in full sometime in 1944 and then she, herself, was ultimately scrapped in 1945.
The success of the I-400 class in their intended roles is, thusly, left to the imagination of the reader.