The I-400 class was the largest submarine class built before the age of nuclear-powered submarines. Japan constructed these enormous vessels during World War II to carry aircraft across long distances and attack targets far from Japanese bases.
Each submarine could carry three Aichi M6A Seiran floatplanes. The aircraft were stored in a watertight hangar on the forward deck and launched using a catapult after assembly. This combination of submarine and aircraft carrier functions gave the class an unusual strategic role.
The I-400 class was designed for operations against the Panama Canal, although the planned mission changed before the submarines could carry it out. I-400 and I-401 surrendered in 1945 and were later inspected by the United States Navy before being scuttled.
The class is distinct from the Typhoon class, which is generally recognized as the largest submarine class ever built overall. The I-400 class holds the earlier pre-nuclear distinction.