A Maritime patrol aircraft, also simply patrol aircraft, or by the older term patrol bomber, is a fixed-wing aircraft designed to operate for long durations over water in maritime patrol, anti-shipping, anti-submarine and search and rescue roles.
The first aircraft that would now be identified as patrol bombers were flown by the Royal Naval Air Service during World War I, primarily on anti-submarine patrols. At first blimps were the only aircraft capable of staying aloft for the long periods of time (as much as 10 hours) need by the patrols whilst carrying a useful payload. Shorter-range patrols were mounted by adapted bombers such as the Sopwith 1 1/2 Strutter. Later in the war aeroplanes were developed specifically for the role. These were usually large floatplanes such as the Short 184 or flying boats such as the Felixstowe F.2.
Many of the World War II patrol aircraft were converted from long-range bombers or airliners, with notable conversions being Germany's Focke-Wulf Fw 200 and the widespread use of B-24 Liberators during World War II to patrol the areas between Iceland and Greenland and attack German submarines. Purpose-built aircraft were also used, including Britain's Short Sunderland and the American PBY Catalina.
After the war the patrol role was generally taken over by conversions of civilian airliners that had range and performance better than most WWII bombers. The latest jet-powered bombers of the 1950s did not have the endurance needed for long, over-water cruising, and more importantly did not have the low loitering speed necessary for anti-submarine operations. The main threat to the United States throughout the 1960s and into the 1980s were Soviet submarines, which were found primarily with the use of sonobuoys, and which then would be attacked with homing torpedoes.
Today the threat of a large scale submarine attack is remote, and many forces are downsizing their patrol aircraft fleets. Those still in service are used primarily for counter-smuggling and Exclusive Economic Zone enforcement duties.
A maritime patrol aircraft typically carries a dozen or so crew members, including relief flight crews, to effectively operate the equipment for up to 12 hours at a time.