The procedure allows the receiving aircraft to remain airborne longer, extending its range or loiter time on station. A series of air refuelings can give range limited only by crew fatigue and engineering factors such as engine oil consumption. Because the receiver aircraft can be topped up with extra fuel in the air, air refueling can allow a takeoff with a greater payload which could be weapons, cargo, or personnel: the maximum takeoff weight is maintained by carrying less fuel and topping up once airborne. Alternatively, a shorter take-off roll can be achieved because take-off can be at a lighter weight before refueling once airborne. Aerial refueling has also been considered as a means to reduce fuel consumption on long-distance flights greater than 3,000 nautical miles (5,600 km; 3,500 mi). Potential fuel savings in the range of 35-40% have been estimated for long haul flights (including the fuel used during the tanker missions).
Usually, the aircraft providing the fuel is specially designed for the task, although refueling pods can be fitted to existing aircraft designs if the "probe-and-drogue" system is to be used. The cost of the refueling equipment on both tanker and receiver aircraft and the specialized aircraft handling of the aircraft to be refueled (very close "line astern" formation flying) has resulted in the activity only being used in military operations. There is no known regular civilian in-flight refueling activity. Originally employed shortly before World War II on a very limited scale to extend the range of British civilian transatlantic flying boats, and then after World War II on a large scale to extend the range of strategic bombers, aerial refueling since the Vietnam War has been extensively used in large-scale military operations. For instance, in the Gulf War and the Iraqi invasion of Kuwait and the Iraq War, all coalition air sorties were air-refueled except for a few short-range ground attack sorties in the Kuwait area.
Some of the earliest experiments in aerial refueling took place in the 1920s; two slow-flying aircraft flew in formation, with a hose run down from a hand-held fuel tank on one aircraft and placed into the usual fuel filler of the other. The first mid-air refueling, based on the development of Alexander P. de Seversky, between two planes occurred on June 27, 1923, between two Airco DH-4B biplanes of the United States Army Air Service. An endurance record was set by three DH-4Bs (a receiver and two tankers) on August 27–28, 1923, in which the receiver airplane remained aloft for more than 37 hours using nine mid-air refuelings to transfer 687 US gallons (2,600 L) of aviation gasoline and 38 US gallons (140 L) of engine oil. The same crews demonstrated the utility of the technique on October 25, 1923, when a DH-4 flew from Sumas, Washington, on the Canada–United States border, to Tijuana, Mexico, landing in San Diego, using mid-air refuelings at Eugene, Oregon, and Sacramento, California.
Similar trial demonstrations of mid-air refueling technique took place at the Royal Aircraft Establishment in England and by the Armée de l'Air in France in the same year, but these early experiments were not yet regarded as a practical proposition, and were generally dismissed as stunts.
As the 1920s progressed, greater numbers of aviation enthusiasts vied to set new aerial long-distance records, using inflight air refueling. One such enthusiast, who would revolutionize aerial refueling was Sir Alan Cobham, member of the Royal Flying Corps in World War I, and a pioneer of long-distance aviation. During the 1920s, he made long-distance flights to places as far afield as Africa and Australia and he began experimenting with the possibilities of in-flight refueling to extend the range of flight.
Cobham was one of the founding directors of Airspeed Limited, an aircraft manufacturing company which went on to produce a specially adapted Airspeed Courier that Cobham used for his early experiments with in-flight refueling. This craft was eventually modified by Airspeed to Cobham's specification, for a non-stop flight from London to India, using in-flight refueling to extend the plane's flight duration.
Meanwhile, in 1929, a group of U.S. Army Air Corps fliers, led by then Major Carl Spaatz, set an endurance record of over 150 hours with the Question Mark over Los Angeles. Between June 11 and July 4, 1930, the brothers John, Kenneth, Albert, and Walter Hunter set a new record of 553 hours 40 minutes over Chicago using two Stinson SM-1 Detroiters as refueler and receiver. Aerial refueling remained a very dangerous process until 1935 when brothers Fred and Al Key demonstrated a spill-free refueling nozzle, designed by A. D. Hunter. They exceeded the Hunters' record by nearly 100 hours in a Curtiss Robin monoplane , staying aloft for more than 27 days.