I-400-class submarine

	I-401, with its long plane hangar and forward catapult
Class overview
Operators	Imperial Japanese Navy
Cost	28,861,000 JPY in 1942
Built	18 January 1943–24 July 1945
In commission	1944–1945
Planned	18
Completed	3
Canceled	15
Retired	3
General characteristics
Type	Submarine aircraft carrier
Displacement	5,223 long tons (5,307 t), surfaced; 6,560 long tons (6,670 t), submerged;
Length	122 m (400 ft)
Beam	12.0 m (39.4 ft)
Draft	7.0 m (23.0 ft)
Propulsion	4 diesels: 1,680 kW (2,250 hp) each, surface; 2 electric motors: 1,600 kW (2,100 hp) each, submerged;
Speed	18.7 kn (34.6 km/h; 21.5 mph), surfaced; 6.5 kn (12.0 km/h; 7.5 mph), submerged;
Range	43,123 mi. (69,400km)
Test depth	100 m (330 ft)
Complement	144 officers and men
Armament	3 M6A1 Seiran floatplane; 8 × 533 mm forward torpedo tubes; 1 × 14 cm/40 11th Year Type naval gun; 3 × 25 mm triple-mounted Type 96 autocannon (9 barrels total); 1 × single-mounted 25 mm Type 96 autocannon;

The I-400-class submarine (伊四百型潜水艦, I-yon-hyaku-gata sensuikan) Imperial Japanese Navy (IJN) submarines were submarine aircraft carriers able to carry three Aichi M6A Seiran aircraft underwater to their destinations. They were designed to surface launch their planes, then quickly dive again before they were discovered. They also carried torpedoes for close-range combat.

At 122 m (400 ft), the three I-400s were the largest submarines of World War II, with the final completed submarine being finished roughly a month before the end of the war. They remained the largest submarines ever built until the construction of nuclear ballistic missile submarines in the 1960s. The official designation was the Sentoku type submarine (潜特型潜水艦, Sen-Toku-gata sensuikan), "Sentoku" being the abreviation of Sensuikan Toku (潜水艦特, Submarine Special).

The I-400 class was designed with the range to travel anywhere in the world and return. A fleet of 18 boats was planned in 1942, and work started on the first in January 1943 at the Kure, Hiroshima, arsenal. Within a year the plan was scaled back to five, of which only three (I-400 at Kure, and I-401 and I-402 at Sasebo) were completed.