The Whitehead No.21 was the aircraft that aviation pioneer Gustave Whitehead claimed to have flown near Bridgeport, Connecticut on August 14, 1901.

A description and photographs of Whitehead's aircraft appeared in Scientific American in June 1901, stating that the "novel flying machine" had just been completed, and "is now ready for preliminary trials". The flight was reported in the August 18, 1901 issue of the Bridgeport Sunday Herald and was subsequently reprinted in other newspapers.Photographs exist showing the aircraft on the ground, but there are no photographs known of the aircraft in flight. A drawing of the aircraft in flight accompanied the Sunday Herald article. The No.21 was a monoplane powered by two enginesone for the wheels during the ground run, the other driving the propellers for flight.

Mainstream aviation scholars dispute the flight; in 1980, C.H. Gibbs-Smith called the story a "flight of fancy", but an editorial written for the 2013 100th anniversary edition of Jane's All the World's Aircraft credits Whitehead as the first man to build and fly a powered heavier-than-air flying machine.

Aviation is the activities surrounding mechanical flight and the aircraft industry. Aircraft includes fixed-wing and rotary-wing types, morphable wings, wing-less lifting bodies, as well as lighter-than-air craft such as hot air balloons and airships.

Aviation began in the 18th century with the development of the hot air balloon, an apparatus capable of atmospheric displacement through buoyancy. Some of the most significant advancements in aviation technology came with the controlled gliding flying of Otto Lilienthal in 1896; then a large step in significance came with the construction of the first powered airplane by the Wright brothers in the early 1900s. Since that time, aviation has been technologically revolutionized by the introduction of the jet which permitted a major form of transport throughout the world.