Japanese submarine I-51
Following World War I, the Imperial Japanese Navy General Staff began to re-consider submarine warfare as an element of fleet strategy. Before the war, the Japanese Navy regarded submarines as useful only for short-range coastal point defense. However, based on the success of the Imperial German Navy in deployment of long-range cruiser-submarines for commerce raiding Japanese strategists came to realize possibilities for using the weapon for long range reconnaissance, and in a war of attrition against an enemy fleet approaching Japan. Procurement for a large, long-range Japanese submarine was authorized in fiscal 1918 under the Eight-six fleet program, under the designation S22.
"Project S22" was based on the latest Royal Navy design, the British K class submarine. Japanese ties to Great Britain via the Anglo-Japanese Alliance were still strong. "Project S22" was laid down at Kure Naval Arsenal on 6 April 1921, launched on 29 November 1921 and completed on 20 June 1924.
Aichi M6A
From the late 1920s, the Imperial Japanese Navy had developed a doctrine of operating floatplanes from submarines to search for targets.
In December 1941, Commander-in-Chief of the Japanese Combined Fleet, Admiral Isoroku Yamamoto, proposed constructing a large fleet of submarine aircraft carriers (also designated STo or "sen-toku" — special submarine) whose purpose was to mount aerial attacks against American coastal cities. The submarines would surface to launch their aircraft by catapult, submerge to avoid detection, then surface again to retrieve the aircrews who would ditch their planes nearby. By June 1942, the plan was to build a fleet of eighteen such subs. This was later cut to nine, then five and finally just three as Japan's wartime fortunes declined.