In 1966, the JMSDF awarded the company a contract to further develop these ideas into an ASW patrol aircraft. Two prototypes were built under the designation PS-X and flight tests began on October 5, 1967, leading to an order for production under the designation PS-1 in 1969. Between 1971 and 1978, the JMSDF ordered 21 of these aircraft, and operated them until 1989 when they were phased out and replaced with P-3 Orions.
The PS-1 had not been in service long before the JMSDF requested the development of a search-and-rescue variant. The deletion of the PS-1's military equipment allowed for greater fuel capacity, workable landing gear, and rescue equipment. The new variant, the US-1A, could also quickly be converted for troop-carrying duties. First flown on October 15, 1974, it was accepted into service the following year, and eventually 19 aircraft were purchased. From the seventh aircraft on, an uprated version of the original engine was used, but all aircraft were eventually modified to this US-1A standard.
With the US-1A fleet beginning to show its age, the JMSDF attempted to obtain funding for a replacement in the 1990s, but could not obtain enough to develop an entirely new aircraft. Therefore, in 1995, ShinMaywa began plans for an upgraded version of the US-1A, the US-1A kai (US-1A 改 - "improved US-1A"). This aircraft features numerous aerodynamic refinements, a pressurised hull, and more powerful Rolls-Royce AE 2100 engines. Flight tests began on December 18, 2003. The JMSDF purchased up to 14 of these aircraft, around 2007 and entered service as the ShinMaywa US-2.
