A fleein boat is a fixed-winged seaplane with a hull, allowing it to land on water, that usually has no type of landing gear to allow operation on land. It differs from a floatplane as it uses a purpose-designed fuselage which can float, granting the aircraft buoyancy. Flying boats may be stabilized by under-wing floats or by wing-like projections (called sponsons) from the fuselage. Fleein boats were some of the largest aircraft of the first half of the 20th century, exceeded in size only by bombers developed during Warld War II. Their advantage lay in using water instead of expensive land-based runways, making them the basis for international airlines in the interwar period. They were also commonly used for maritime patrol and air-sea rescue.
Their use gradually trailed off after Warld War II, partially because of the investments in airports during the war. In the 21st century, fleein boats maintain a few niche uses, such as dropping water on forest fires, air transport around archipelagos, and access to undeveloped areas. Many modern seaplane variants, whether float or fleein boat types, are convertible amphibious aircraft where either landing gear or flotation modes may be used to land and take off.