From 1962 onwards, the UF-XS performed numerous test flights, demonstrating the improved features which enabled a flying boat to both land and take-off from the open ocean; these tests were closely followed and critiqued by the Japanese military. Over time, the UF-XS continued to be modified to improve its stability and other key performance criteria. In 1966, the Japan Maritime Self-Defense Force (JMSDF) awarded Shin Meiwa a contract to further develop its design to produce a patrol aircraft capable of the ASW mission; accordingly, two further prototypes were constructed under the designation "PS-X". 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. 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.