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.
Yokosuka E6Y
The Imperial Japanese Navy was a pioneer in naval aviation, starting as early as 1912 with the purchase of two floatplanes from Britain and one from the United States. By December 1922, Japan had completed "Hōshō", which vies with as the first ship purpose-designed for aircraft operations. Alongside that development, the Navy also looked at aircraft as a way to extend the operational reach of their large submarine force. Aware of the challenge of operations in the large expanse of the Pacific Ocean, the Navy was particularly looking at ways to improve their reconnaissance capability and saw submarine-based aircraft as a complement to land--based patrol. They acquired a Caspar U.1 from the United States and a Parnall Peto from Britain, both early submarine-based reconnaissance aircraft. The two aircraft formed the basis for two prototype Japanese aircraft built for submarine-based reconnaissance, the 1-Go based on the former, while the latter heavily influenced the 2-Go.