In-flight refueling was one way to set airborne endurance records. The Atlantic-Fokker X-2A "Question Mark" stayed aloft for seven days in January of 1929, refueling 42 times in the air. Because radio communication was unreliable at this time, the pilots in the two planes communicated by notes dropped to the ground and by hand signals, flashlight signals, ground panels, and messages written on blackboards carried in the planes.

Credits - U.S. Air Force Museum