The PS-1 anti-submarine warfare (ASW) variant is a flying boat which carried its own beaching gear on board, while the search-and-rescue (SAR) orientated US-1A is a true amphibian.

The company, believing that their design was capable of regular use upon the open sea, petitioned the Japanese military to acquire the type as a maritime patrol aircraft (MPA).

Shin Meiwa were keen to develop additional variants and derivative aircraft, including substantially larger designs which they had studied, but many of these ambitions remained as paper projects only.

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.

[2] 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.

[2][3] 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.

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.

[2] 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.

For its part, Shin Meiwa made efforts to commercialise design elements of the aircraft, such as its hydraulics and engine control systems; it exported its rough-sea roll-damping technology to other seaplane companies.

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[2]) 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.