Question Mark ("?") was a modified Atlantic-Fokker C-2A transport airplane of the United States Army Air Corps. In 1929, commanded by Major Carl A. Spaatz, it was flown for a flight endurance record as part of an experiment with aerial refueling. Question Mark established new world records in aviation for sustained flight (heavier-than-air), refueled flight, sustained flight (lighter-than-air), and distance between January 1 and January 7, 1929, in a nonstop flight of 151 hours near Los Angeles, California.

Following the record-setting demonstration, the C-2A was returned to transport duties. In 1931, more powerful engines replaced those used in the endurance flight and it was redesignated as a C-7 transport. The aircraft was damaged beyond economical repair in 1932, when it crash-landed in Texas after running out of fuel, and was scrapped.

The flight demonstrated the military application of the concept, but while it inspired numerous efforts to set even greater endurance records, development of a practical in-flight refueling system was largely ignored by the world's air forces before World War II. Civilian development of aerial refueling in Great Britain was more successful, but in the end, it, too, was disregarded. 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, when two Boeing-built de Havilland DH-4Bs of the United States Army Air Service accomplished the feat over San Diego's Rockwell Field. Subsequently, the same group of airmen established an endurance record of remaining aloft for more than 37 hours in August 1923, using nine aerial refuelings. In June 1928, a new endurance record of more than 61 hours was established in Belgium by Adjutant Louis Crooy and Sgt. Victor Groenen, also using aerial refueling.

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. 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. Quesada submitted the plan to Capt. Ira C. Eaker, an aide to Assistant Secretary of War for Air F. Trubee Davison, who had also been on the April mission. Their mutual interest in air-to-air refueling led Eaker to forward it to Maj. Gen. James E. Fechet, Chief of the Air Corps. Both Fechet and Davison approved the project on the condition that it demonstrate a military application and not just as a publicity stunt. Overall command of the project was given to Major Carl A. Spaatz the Assistant G-3 for Training and Operations in Fechet's office, who was on orders to take command of the 7th Bombardment Group at Rockwell Field.

A new Atlantic-Fokker C-2A transport, serial number 28-120, was selected for the project. Assigned to the Air Corps Detachment, Bolling Field at Bolling Field, it was flown to Middletown Air Depot, Pennsylvania, for modifications. 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. A C-2 variant nicknamed the Bird of Paradise had made the first transpacific flight to Hawaii the year before and proven the capability of the design. The C-2A had an internal fuel capacity of 192-US-gallon (730 L) in a pair of wing tanks, and for the project, two 150-US-gallon (570 L) tanks were installed in the cargo cabin. A hatch was cut in the roof of the C-2 behind the wing for transfer of the fuel hose and passage of supplies from the tanker to the receiver. The 72-octane aviation gasoline would be received in 100-US-gallon (380 L) increments of about 90 seconds' duration.

A 45-US-gallon (170 L) tank was used to provide engine oil to the three motors, replenished by in-flight deliveries of 5-US-gallon (19 L) cans of Pennzoil triple-extra-heavy lowered on slings. A copper tubing system was installed in an attempt to adequately lubricate the rocker arms of the engines in-flight. 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. To reduce propeller noise, the two wing engines were mounted with Westinghouse twin-blade Micarta propellers, while the nose engine used a Standard three-blade steel propeller.

As word of the project spread, its members were continually being asked how long they expected to remain aloft. Their responses were generally to the effect: "That is the question." 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.