Hydrogen-powered plane carries human crew for first time.
The small, propeller-driven craft, developed by aviation giant Boeing, made three short flights at an airfield south of Madrid, the company said.
It was powered by hydrogen fuel cells, which produce only heat and water as exhaust products.
The tests could pave the way for a new generation of greener aircraft, the company said.
Boeing's chief technology officer John Tracy said the flights were "a historical technological success" and "full of promises for a greener future".
Three test flights of the two-seater aircraft took place in February and March at an airfield at Ocana, south of Madrid. The plane was modified to include a hybrid battery and fuel cell system used to power an electric motor coupled to a conventional propeller.
Hydrogen-powered planes have been flown before, but never with a human pilot onboard.