A Fokker C-2A is refueled in flight by a modified Douglas C-1 transport aircraft during an operation dubbed "Question Mark" in 1929. A large question mark was painted on each side of the receiver aircraft's fuselage, intended to "provoke wonder at how long the aircraft could remain airborne," according to Air Force historians. The Douglas C-1 aircraft were transformed into aerial refueling tankers by installing two 150-gallon tanks for offloading and a refueling hose that passed through a hatch cut in the floor of the aircraft. During the operation, which began on New Year's Day in 1929 and ended 150 hours and 40 minutes later on Jan. 7, the two Douglas refueling aircraft passed 5,660 gallons of fuel, completing 43 sorties, 12 of which occurred at night. (U.S. Air Force photo)