However, Spaatz, two decades after the flight of the Question Mark, became head of the United States Air Force and using the British system as a starting point, implemented in-flight refueling on a worldwide operational basis.

The first complete in-flight refueling between two aircraft took place on June 27, 1923,[n 1] when two Boeing-built de Havilland DH-4Bs of the United States Army Air Service accomplished the feat over San Diego's Rockwell Field.

[1] Second Lt. Elwood R. Quesada, an engineer of the U.S. Army Air Corps stationed at Bolling Field in Washington, DC, nearly crashed from lack of fuel in April 1928 while participating in a long-range rescue mission to Labrador.

[2] Working with a U.S. Marine Corps aviator from Anacostia Naval Air Station, he devised a plan to break the Belgians' record using aerial refueling.

[4] The C-2A was an American-built military version of the Fokker F.VIIa-3m Trimotor, a high-wing monoplane with a gross weight of 10,395 pounds (4,715 kg), re-engined with three Wright R-790 motors producing 220 horsepower (160 kW) each.

Doorways were cut on each side of the cockpit and catwalks built on the wings to enable mechanic Roy Hooe to access the engines for emergency maintenance.

A large question mark was painted on each side of the fuselage to provoke interest in the endurance flight, prompting the nickname of the plane.

The operation was scheduled to begin Tuesday, January 1, 1929, at Los Angeles, California, to take advantage of weather conditions and to generate publicity while refueling by overflying the 1929 Rose Bowl football game played that day in Pasadena.

[11] Question Mark took off from Van Nuys at 7:26 am on New Year's Day 1929 with Eaker at the controls, carrying only 100 US gallons (380 L) of fuel to save takeoff weight.

[8] Aboard the Question Mark, either Halverson or Quesada did most of the piloting during cruising flight while Eaker monitored the throttles for smoothest engine performance.

During refuelings, Eaker and Halverson manned the controls, Spaatz and Quesada supervised the fuel exchange, and Hooe operated a "wobble" pump.

Spaatz climbed on a platform below the open hatch, and wearing rain gear and goggles for protection against fuel spills, grounded the hose and then placed it in a receptacle mounted in the upper fuselage.

[13] Made from a bucket with a sloped floor, the receptacle had connections to the two extra fuel tanks, and at Spaatz's signal, Solter opened the valve.

However, an electric stove to heat food was eliminated to save weight, and hot meals were sent aloft by the refuelers, including a turkey dinner on New Years Day prepared by a church in Van Nuys.

After the existing endurance record was surpassed on Thursday evening, January 3, the support crew sent up cheese, figs, olives, and five jars of caviar for an in-flight celebration.

[3] Fearing that chemical burns from the gasoline might force him to parachute from the airplane to seek medical treatment, Spaatz ordered Eaker to continue the flight regardless.

[13] Maintaining contact formation became more difficult as the weight of the planes changed during transfer, especially since the refueling pilot could not observe the Question Mark.

On a second attempt westbound, his engine quit over Utah when dirt clogged the fuel line, forcing him to crash land in the mountains near Salt Lake City.

At the end of 1929, the record stood at over 420 hours, established by Dale "Red" Jackson and Forest E. "Obie" O'Brine in the Curtiss Robin Greater St.

On May 21, 1929, during annual maneuvers, a Keystone LB-7 piloted by Moon took off from Fairfield Air Depot in Dayton, Ohio, on a simulated mission to New York City via Washington, DC.

Plans were for the bomber to be refueled in flight several times, drop a flash bomb over New York harbor, then return to Dayton nonstop, again by way of Washington.

While attempting an air refueling en route from Dayton to Washington, icing forced the tanker to land in Uniontown, Pennsylvania, where it got stuck in mud.

[21] The next day, the tanker joined the bomber and both flew to New York, where they made a public demonstration of air refueling and four dry runs.

[24] Solter, a pursuit pilot, was killed in an accident flight testing an all-metal trainer at Randolph Field, Texas, in September 1936.

The aircraft was severely damaged trying to land at Davenport Auxiliary Field, 4 miles (6.4 km) north of the base,[27][28][n 8] and was surveyed (scrapped) in 1934.

"[3] In his report to Fechet, Spaatz concluded that aerial refueling was both safe and practical, and that for bombardment operations it made the potential radius of action of a bomber almost unlimited while allowing it to carry heavier bomb loads because the weight of fuel at takeoff could be reduced.

As a result, the only immediate effect of the flight was to start a craze among aviators that one official USAF history characterized as "aeronautical flagpole sitting.

"[21][n 9] The flight of the Question Mark did spark an interest in aerial refueling by the Royal Air Force, seeking to minimize takeoff weights of its bombers and reduce wear-and-tear on its grass airfields, but after a series of experiments between 1930 and 1937, they abandoned testing.

In 1939, trials to perform aerial refueling of Short Empire flying boats began, but were suspended after 16 flights following the outbreak of World War II.

[38][n 10] In January 1948, 19 years after he commanded the Question Mark project, USAF Chief of Staff Spaatz made aerial refueling the Air Force's top strategic priority.