The "Zephyr" drone, developed by British defense technology company QinetiQ in Farnborough, England, took off from the U.S. Army's Yuma Proving Ground in Arizona July 9, CNN reported.
It was still flying at the weekend, and its designers say they hope it can remain airborne for another seven days, the U.S. news network said.
"It's extremely exciting," Zephyr program director Jon Saltmarsh said. "What we now have is an eternal plane. It has the same amount of fuel at the start of one day as it does at the start of another."
The Zephyr project was conceived in 2001 and funded by the United Kingdom's Ministry of Defense.
Saltmarsh says the aircraft demonstrates genuine military utility.