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First in-flight refueling exercise -

This photograph captures the historic moment when the first ever in-flight refueling exercise was carried out, on November 12, 1921. The image depicts Wes May climbing from a Standard J to a Curtiss JN-4C Jenny with a 50-pound (22.5-liter) fuel can strapped to his back.

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Onboard fuel -

On October 5, 1922, US Army Air Service pilots Oakley G. Kelly and John A. Macready set a world endurance record of 35 hours, 18 minutes, and 30 seconds in their Fokker T-2 airplane, flying from New York to San Diego. Had they not run low on aviation gasoline, only personal fatigue or mechanical difficulty would have curtailed their flight. The pair are pictured next to their aircraft with the drums of fuel they carried in order to complete the non-stop transcontinental flight.

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First successful aerial refueling -

History was made on June 27, 1923, at Rockwell Field, San Diego, when one DH-4B biplane carrying pilots Virgil Hine and Frank W. Seifert successfully passed via a hose fuel to another DH-4B flown by Lowell H. Smith and John P. Richter. This was the world's first successful aerial refueling maneuver. Image: US Army

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Endurance competitions -

The ability to refuel midair spawned numerous flight endurance competitions through the 1920s. Here, American aviators Loren Mendell and R.B. Reinhart refuel their biplane above Culver City, California, after setting a new record in 1929, staying aloft for 246 hours and 43 minutes.

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Non-stop flights -

English aviator Alan Cobham (1894–1973) was a recognized pioneer of long-distance aviation and aerial refueling. In 1934, he completed a non-stop flight from London to India, using in-flight refueling delivered by a Handley Page Type W to extend his Airspeed Courier's flight duration (pictured).

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Flight Refueling Ltd -

Also in 1934, Cobham (pictured) founded Flight Refueling Ltd (FRL). In 1939, British airline Imperial Airways performed several non-stop crossings of the Atlantic using equipment provided by FRL.

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Commercial applications -

Cobham envisioned his refueling system as purely facilitating long-range transoceanic commercial aircraft flights such as those made by the Empire flying boat fleet. In this photograph, a Short flying boat is being refueled in air by Handley Page Harrow.

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Wartime development -

During the Second World War, air refueling was planned for use on RAF Lancaster bombers in the Pacific as part of Tiger Force, deploying the looped hose refueling method developed by FRL. However, the conflict ended before they saw operational use.

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Refueling in the jet age -

Post-war, the Lancaster was put to good use as a modified tanker. In trials, and employing the much improved probe-and-drogue refueling system, a modified RAF Gloster Meteor jet fighter with a nose-mounted probe (pictured) was successfully refueled midair.

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First non-stop circumnavigation of the world -

In 1949 as the Cold War began to heat up, the US Air Force Boeing B-50 Superfortress Lucky Lady II was successfully refueled by grappled-line looped-hose during the first non-stop circumnavigation of the world by air. Besides representing a technological breakthrough, the event was designed to show Moscow that all targets were now within range of Strategic Air Command bombers.

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First four-point refueling operation -

By the 1950s, aerial refueling was being used almost exclusively by military aircraft. As they were withdrawn from service, some commercial flying boats were converted into tankers for inflight refueling roles. In 1956, a modified Convair R3Y Tradewind flying boat successfully refueled four Grumman F9F Cougars simultaneously in flight using the probe-and-drogue method. Image: US Navy

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Ground facilities -

The United States Air Force made research and development of aerial refueling a top priority. To accommodate the growing fleet of tankers, two dedicated air refueling units had been formed: the 43d Air Refueling Squadron at Davis-Monthan Air Force Base, Arizona, and the 509th Air Refueling Squadron at Walker Air Force Base, New Mexico.

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First use of refueling during combat -

On July 6, 1951, the first use of aerial refueling in combat took place during the Korean War. This effectively doubled the range of fighter-bombers such as the F-84 and RF-80.

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The flying boom -

By now the Americans had perfected an alternative way of delivering fuel, using a new system called the flying boom.

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Boeing KC-97 Stratofreighter -

In effect a retractable pipeline, once deployed from the tanker aircraft the flying boom could extend, telescope-like, to twice its usual length. Furthermore, the boom transferred fuel at 700 gallons (2,649 liters) per minute—nearly triple that of the probe-and-drogue. Pictured is a Boeing KC-97 Stratofreighter, the precursor to the more advanced KC-135.

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First jet-powered refueling tanker -

Maintaining velocity with jet bombers at high altitude while loaded with fuel required a special type of jet tanker. The Boeing KC-135 Stratotanker, the first jet-powered refueling tanker, entered service with the USAF in 1957 and was used extensively in the Vietnam War.

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Use during the Vietnam War -

Pictured are F-105 Thunderchief fighter-bombers being refueled by a KC-135 during a combat mission over Vietnam in 1966.

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KC-135 in action -

Pictured: a B-52 Stratofortress bomber approaches a KC-135 aerial tanker for refueling prior to a strike in Vietnam.

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KC-135 boom operator -

Refueling via a flying boom requires a dedicated boom operator. The boom operator also controls the refueling drogue, a basket attached to a flexible hose that trails the tanker, when using the probe-and-drogue system.

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Round-the-clock capability -

Pictured: a C-17 Globemaster III receives fuel from a KC-135 during night operations. Image: US Air Force

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Air-to-air view -

Pictured: an air-to-air view of a KC-135 Stratotanker aircraft preparing to refuel an F-15 Eagle aircraft from the 18th Tactical Fighter Wing, as seen from the cockpit of the F-15.

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McDonnell Douglas KC-10 Extender -

The KC-135 is one of nine military fixed-wing aircraft with over 60 years of continuous service with the USAF. In 1980, the KC-135 was supplemented by the McDonnell Douglas KC-10 Extender.

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KC-10 Extender boom operator -

The KC-10 Extender boom operator is seated in the rear of the aircraft with a wide window for monitoring refueling rather than prone as in the KC-135.

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Mixed fueling system -

The KC-10's mixed refueling system of hose-and-drogue and flying boom allows it to refuel the aircraft of the US Air Force, Navy, Marine Corps, and allied forces. The tanker saw combat during Operation Desert Storm in 1991.

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High performance -

Refueling can be performed at an altitude up to 35,000 feet (10,668 m) while cruising at speeds between 180 knots and 325 knots.

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Boeing KC-46 Pegasus -

The Boeing KC-46 Pegasus, modeled on the 767 jet airliner, is set to replace older KC-135s. The USAF intends to procure 179 Pegasus tankers by 2027. Image: US Air Force

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Refueling helicopters -

Rotorcraft also benefit from midair refueling. Here, a French Air Force EC725 is refueled by a drogue-equipped Lockheed HC-130 during Operation Angel Thunder in 2014.

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First operational tests -

The first operational tests refueling helicopters midair took place in Southeast Asia on June 21, 1967. A HC-130P Hercules refueled a Sikorsky HH-3E "Jolly Green Giant" rescue helicopter over the South China Sea.

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Dangerous maneuver -

Extra care needs to be taken when performing helicopter aerial refueling operations. Their rotor blades can accidentally contact the drogue or fuel hose, the results of which can be catastrophic.

Sources: (National Museum of the USAF) (Fédération Aéronautique Internationale) (Air Mobility Command) (Aviation Stack Exchange) 

See also: The most tragic plane crashes in US history.

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