Initially, the company focused on smaller efforts, such as subcontracting work, the production of drop tanks, and performing airframe overhauls of both Japanese and American aircraft, such as the US Navy's Martin P5M Marlin flying boats.

However, senior figures, such as chief aircraft designer Shizo Kikuhara and founder Ryuzo Kawanishi were keen to pursue projects of a greater scope.

[3] Unlike most seaplanes, they held the ambitious aim of producing an aircraft that could land upon rough seas and encounter little impact from waves and spray.

[3][4] Two years later, Kikuhara, who now headed up the company's Amphibian Development Division, was lobbying the Japanese Defense Agency to consider the adoption of a flying boat to meet the nation's requirement for an anti-submarine warfare (ASW) patrol aircraft.

Crucial support came from the US Navy, who were keen to see Japan's ASW capabilities expand to help track and contain the growing Soviet submarine presence in the Pacific.

In addition to Shin Meiwa, other Japanese companies, such as Fuji Heavy Industries and NIPPI Corporation, also played major roles in the PS-X's development.

[3] The adaptions resulted in significant seaworthiness improvements; during tests conducted in the Kii Channel during 1968, the PS-X successfully landed amid formidable four-meter waves, despite these being in excess of its design goal of three meters.

During the 1990s, by which point the US-1A fleet was beginning to show its age, the JMSDF attempted to obtain funding towards acquiring a replacement, but could not secure enough to develop an entirely new aircraft.

Therefore, during 1995, ShinMaywa (as Shin Meiwa had been renamed, reportedly so that the name would be easier to pronounce for non-Japanese speakers[3]) set about planning to produce an upgraded version of the US-1A, initially referred to as the US-1A kai (US-1A 改 - "improved US-1A").

This aircraft features numerous aerodynamic refinements and modernised systems, along with a pressurised hull, and the adoption of more powerful Rolls-Royce AE 2100 engines.