A flying boat is a specialised form of aircraft that is designed to take off from and land on water, using its fuselage as a floating hull. Such aircraft are sometimes stabilised on water by underwing floats or by wing-like projections from the fuselage. It is the use of the fuselage to provide the main buoyancy of the aircraft which distinguishes flying boats from floatplanes, which use one or more floats attached below the fuselage or the wings to keep the fuselage clear of the water.
Flying boats were some of the largest aircraft of the first half of the 20th century. Their ability to alight on water allowed them to break free of the size constraints imposed by general lack of large, land-based runways, and also made them important for maritime patrol and air-to-sea rescue, capabilities put to great use in World War II. Following World War II, their use gradually tailed off, with many of the roles taken over by land aircraft types.
In the 21st century, flying boats maintain a few niche uses, such as for dropping water on forest fires and for air transport around archipelagos.
The American pioneer aviator Glenn Curtiss had been built experimental floatplanes before 1910. He joined with English naval captain John Cyril Porte to design a flying boat that could win the cash prize offered by the British Daily Mail newspaper for the first aerial crossing of the Atlantic ocean. Porte developed a practical hull design with the distinctive 'notch' which could be married to Curtiss' airframe and engine design, creating the Atlantic, or Type A flying boat. The innovation of the notch enabled the craft to overcome suction from the water and break free for flight. The resulting aircraft would be large enough to carry sufficient fuel to fly long distances and could berth alongside ships for refuelling. In 1911 Curtiss unveiled a development of his floatplane and landplane model D, this time fitted with a hull, and designated as the Model E. The outbreak of war interrupted their plans, as Porte rejoined the Royal Navy.
In 1913, the boat building firm J. Samuel White of West Cowes on the Isle of Wight set up a new aircraft division and produced a flying boat. This was displayed at the London Air Show at Olympia in 1913 . In that same year, a collaboration between the S.E. Saunders boatyard of East Cowes on the Isle of Wight and the Sopwith Aviation Company produced their "Bat Boat", an aircraft with a consuta laminated hull that could operate from land or on water  . The "Bat Boat" completed several landings on sea and on land and was duly awarded the Mortimer Singer Prize . It was the first all-British aeroplane capable of making six return flights over five miles within five hours.
From 1914 Curtis produced his "America" flying boat, several examples of which were acquired by the Royal Naval Air Service and tested at their Seaplane Experimental Station, now run by Porte. Porte developed an improved hull, resulting in the Felixstowe F.1 and its larger derivatives, used for coastal patrols and hunting U-boats.
The Curtiss Aeroplane and Motor Company independently developed its designs into the small model 'F', the larger model 'K' several of which were sold to the Russian Naval Air Service, and the Model 'C' for the US Navy. Curtiss among others also built the Felixstowe F5 as the Curtiss F5L, based on the final Porte hull designs and powered by American Liberty engines.
In the 1930s, flying boats made it possible to have regular air transport between the U.S. and Europe, opening up new air travel routes to South America, Africa, and Asia. Foynes, Ireland and Botwood, Newfoundland and Labrador were the termini for many early transatlantic flights. Where land-based aircraft lacked the required airfields to land, flying boats could stop at small island, river, lake or coastal stations to refuel and resupply. The Pan Am Boeing 314 "Clipper" planes brought exotic destinations like the Far East within reach of air travellers and came to represent the romance of flight.
In 1923, the first British commercial flying boat service was introduced with flights to and from the Channel Islands. The British aviation industry was experiencing rapid growth. The Government decided that nationalization was necessary and ordered five aviation companies to merge to form the state-owned Imperial Airways of London (IAL). IAL became the international flag-carrying British airline, providing flying boat passenger and mail transport links between Britain and South Africa using aircraft such as the Short S.8 Calcutta.
In 1928, a new world achievement in aviation attracted the attention of the Australian public when four Supermarine Southampton flying boats of the RAF Far-East flight arrived in Melbourne on a circumnavigation and flag-waving mission. The RAF crews were warmly welcomed by the waterside crowds, and the flight was considered proof that flying boats had evolved to become reliable means of long distance transport.
Queensland and Northern Territory Aerial Services, better known as Qantas, had been registered in Brisbane during November 1920. With good levels of public support for the new faster public transport and agreements to carry domestic mail, the outback airline grew. By 1931, Qantas was trialling land plane flights connecting with Imperial Airways services. Mail was now reaching London in just 16 days - less than half the time taken by sea.
Government tenders on both sides of the world invited applications to run new passenger and mail services between the ends of Empire, and Qantas and IAL were successful with a joint bid. A company under combined ownership was then formed, Qantas Empire Airways. The new ten day service between Sydney's Rose Bay and Southampton was such a success with letter-writers that before long the volume of mail was exceeding aircraft storage space. A solution to the problem was found by the British Government, who in 1933 had requested aviation manufacturer Short Brothers to design a big new long-range monoplane for use by IAL. Partner Qantas agreed to the initiative and undertook to purchase six of the new Short S23 'C' class or 'Empire' flying boats.
Delivering the mail as quickly as possible generated a lot of competition and some innovative solutions. A variant of the Short Empire flying boats, Maia and Mercury, was a strange-looking solution where a four-engined floatplane Mercury was fixed on top of Maia, a heavily modified Short Empire flying boat . The idea was to use the larger Maia to get the smaller Mercury (the winged messenger) off the ground at weights that would have been impossible otherwise, so that it could carry sufficient fuel for the trip. Unfortunately this limited the usefulness, and after crossing to New York the Mercury had to be returned by ship. The Mercury was to set a number of distance records before in-flight refuelling was adopted.
Sir Alan Cobham devised a method of in-flight refuelling in the 1930s, so that the Short Empire flying boats serving the transatlantic crossing could be refuelled over Foynes on the River Shannon in Ireland allowing them to carry more fuel than they could take off with, so as to enable them to make the trans-Atlantic flight . A Handley Page H.P.54 Harrow was used as the fuel tanker
The German Dornier Do-X flying boat was noticeably different from its UK and US-built counterparts, using wing-like protrusions from the fuselage called sponsons, pioneered by Claudius Dornier during World War I on his Dornier Rs.I giant flying boat, to stabilise on the water without the need for wing-mounted outboard floats, and perfected on the Dornier Wal in 1924. The enormous Do X was powered by 12 engines and carried 170 persons.  . It flew to America in 1929  crossing the Atlantic via an indirect route. It was the largest flying boat of its time but was severely underpowered and was limited by a very low operational ceiling. Only three were built with a variety of different engines installed, in an attempt to overcome the lack of power. Two of these were sold to Italy.
The military value of flying boats was well recognized and every country bordering on water operated them in a military capacity at the outbreak of the war. They were utilized in various tasks from anti-submarine patrol to maritime search and rescue and gunfire spotting for battleships. Aircraft such as the PBY Catalina, Short Sunderland and Grumman Goose recovered downed airmen and operated as scout aircraft over the vast distances of the Pacific Theater and Battle of the Atlantic during World War II, as well as sinking numerous submarines, and finding enemy ships. The German battleship Bismarck was found during a routine patrol by a PBY Catalina.
The largest flying boat of the war was the Blohm und Voss Bv 238 which was also the heaviest plane to fly during the Second World War.
In November 1939, the structure of Imperial Airways was changed to create British European Airways and British Overseas Airways Corporation with the change being made official in 1 April 1940. BOAC continued to operate flying boat services from the (slightly) safer confines of Poole Harbour during wartime, returning to Southampton in 1947 .
The Hughes H-4 Hercules in development in the U.S. during the war was even larger than the Bv238, but it did not fly until 1947. The "Spruce Goose", as the H-4 was nicknamed, was the largest flying boat ever to fly. That short 1947 hop of the 'Flying Lumberyard' was to be its last however, a victim of post-war cutbacks and the disappearance of its intended mission as a transatlantic transport.
During the Berlin Airlift (which lasted from June 1948 until August 1949) ten Sunderlands and two Hythes were used to transport goods from Finkenwerder on the Elbe near Hamburg to the isolated city, landing on the Havelsee lake beside RAF Gatow until it iced over. The Sunderlands were particularly used for transporting salt, as their airframes were already protected against corrosion from seawater. Transporting salt in standard aircraft risked rapid and severe structural corrosion in the event of a spillage. This is the only known operational use of flying boats within central Europe.
Following the end of World War II, the use of flying boats rapidly declined, though the U.S. Navy continued to operate such aircraft (notably the Martin P5M Marlin) until the early 1970s, even attempting to build a jet-powered seaplane bomber, the Martin Seamaster. Several factors contributed to the decline. The ability to land on water became less of an advantage owing to the considerable increase in the number and length of land based runways, whose construction had been driven by the needs of the allied forces during the Second World War. Further, as the speed and range of land-based aircraft increased, the commercial competitiveness of flying boats diminished, as their design compromised aerodynamic efficiency and speed to accomplish the feat of waterborne takeoff and alighting. Competing with new civilian jet aircraft like the de Havilland Comet and Boeing 707 was impossible.
BOAC continued to operate their flying boat services out of Southampton until November 1950.
Bucking the trend, in 1948, Aquila Airways was founded to serve destinations that were still inaccessible to land based aircraft . This company operated Short S.25 and Short S.45 flying boats out of Southampton on routes to Madeira, Las Palmas, Lisbon, Jersey, Majorca, Marseilles, Capri, Genoa, Montreux and Santa Margherita . The airline ceased operations on 30 September 1958  .
The flying boats of Aquila Airways were also chartered for one-off trips, usually to deploy troops where scheduled services didn't exist or where there were political considerations. Three Aquila flying boats were used during the Berlin Airlift . The longest charter, in 1952, was from Southampton to the Falkland Islands . In 1953 the flying boats were chartered for troop deployment trips to Freetown and Lagos and there was a special trip from Hull to Helsinki to relocate a ships crew .
The technically advanced Saunders-Roe Princess first flew in 1952 and later received a certificate of airworthiness. Despite being the pinnacle of flying boat development, none were sold, despite Aquila Airways reportedly attempting to buy them . Of the three Princess that were built, two never flew and all were scrapped in 1967
Helicopters ultimately took over the flying boat air-sea rescue role.
The shape of the Short Empire was a harbinger of the shape of later aircraft yet to come, and the type also contributed much to the designs of later ekranoplans. However, true flying boats have largely been replaced by seaplanes with floats and amphibian aircraft with wheels. The Beriev Be-200 twin-jet amphibious aircraft has been one of the closest 'living' descendants of the flying-boats of old, along with the larger amphibious planes used for fighting forest fires. There are also several experimental/kit amphibians such as the Volmer Sportsman, Glass Goose, the LSA SeaMax, Aeroprakt A-24, and the Seawind.
The ShinMaywa US-2 (Japanese: 新明和 US-2) are large STOL aircraft designed for air-sea rescue (SAR) work.US-2 is operated by Japan Self Defense Force.