On June 27, 1923, two biplane pilots made history by swapping fuel through a hose while in flight. It was an Army experiment, working to make practical a stunt that wing-walkers had tried—with some success—previously.
Captain Lowell H. Smith and Lieutenant John P. Richter took off in an Airco DH-4B biplane from Rockwell Field in San Diego. An identical second biplane took off shortly after, flown by First Lieutenants Virgil Hine and Frank Seifert. Because both planes were piloted from the front seat, that left the men in the second seats to mess around with fuel lines. The cockpits were all open, without glass shields to complicate things.
Seifert lowered a hose from his plane, flying just a bit higher, and Richter grabbed it. Richter crammed it into his biplane's fuel tank, gave the high sign to Seifert, and the gas started flowing from the higher tank to the lower. It worked, transferring fuel until Seifert cut off the flow and retracted the hose. The teams landed successfully, then celebrated the first true mid-air refueling in aviation history.
Here's film of the event:
Two months later the Army topped that feat by flying three identical biplanes (again Airco DH-4Bs), two of them acting as support tankers for the third, which was again piloted by Smith. They proceeded to repeat the earlier procedure over and over, and ended up keeping Smith's plane flying continuously for 37 hours—an endurance record at the time. (The other two planes took turns landing and refueling throughout.)
This latter attempt was truly intense. In all, the trio of planes transferred 687 gallons of gas and 38 gallons of oil over the day-and-a-half-long flight. (The oil transfer was necessary because engines of the day bled oil at an incredible rate.) A similar—albeit far more automated—midair refueling process is still in use today.
Although these two flights in 1923 are considered the first "true" midair refueling procedures, technically gas was transferred between planes almost two years earlier. In 1921, the trio of Wes May, Frank Hawks, and Earl Daugherty hatched a plan. May was a stuntman and skilled wing-walker, who was capable of wild things like handstands on the wings of planes in the air. Maybe he could transfer gas between two planes in flight?
May strapped a five-gallon tank of gas onto his back and got in the passenger seat of Hawks's Lincoln Standard biplane. They took off and approached a Curtis Jenny, which was piloted solo by Daugherty in the rear seat. While the two planes flew side by side (these pilots were barnstormers), May climbed out of his seat, walked across the Lincoln's wing, and scrambled onto the wing of the Curtis Jenny. He then climbed into the front seat, unscrewed the fuel tank, and carefully poured in the gas. It worked, but it was slow and extraordinarily dangerous. (Plus the procedure was sufficiently slow that it didn't give a practical benefit to the recipient aircraft.)