The Martin P5M Marlin has the distinction of being the last operational flying boat of the US Navy in an era where such amphibious designs were slowly being replaced by landplanes like the P-2. Developed as a successor to the World War II-era Mariner (which was also built by Martin), it retained its predecessor's wing and upper hull with a new lower hull, T-tail with dihedral tailplanes, radar operated turrets, and more powerful engines. Besides its obvious role as a patrol aircraft, many Marlins were used for anti-submarine duties with magnetic anomaly detectors (MAD) and sonobuyos. Although in general the Marlin was overshadowed by land-based designs, it remained in US Navy service until 1966 and also briefly equipped the US Coast Guard before they were handed back as trainers. The only foreign operator was the French Navy which acquired 10 units.

The prototype XP5M-1 was basically a rebuilt Mariner and first flew on 30 May 1948. It was ordered into production as the P5M-1 which led to various sub variants such as the P5M-1S ASW aircraft with MAD equipment, the P5M-1G for the USCG later adopted by the USN as P5M-1T trainers. The redesigned P5M-2 introduced the T-tail and more powerful engines and was similary converted to ASW duties as the P5M-2S and the P5M-2G for the USCG. After 1962, surviving units were redesignated as follows: P-5A (P5M-1), P-5B (-2), and SP-5B (-2S).