This article is about an airplane. For the number 21, see 21 (number).

Number 21 was the name of the aeroplane Gustave Whitehead allegedly flew with on August 14, 1901 - two years before the Wright brothers' first flight.

According to Gustave and the single eyewitness to the event, the monoplane's longest flight was 60 meters (200 feet) above ground for 2.5 kilometers (1.5 miles). However, these claims are contested. Gustave failed to keep a log book and planned for the bulk of his flights to occur at night, or "early dawn", thus limiting witnesses.

Number 21 was a monoplane powered by a 20 hp engine, fueed by acetylene. It achieved takeoff speed by using the engine to power the wheels, as in an automobile; upon takeoff, power was switched to the propeller.