Record Solar Powered Flight
The Zephyr-6, as it is known, stayed aloft for more than three days, running through the night on batteries it had recharged in sunlight.
The flight was a demonstration for the US military, which is looking for new types of technology to support its troops on the ground.
Craft like Zephyr might make ideal platforms for reconnaissance.
They could also be used to relay battlefield communications.
Chris Kelleher, from UK defence and research firm QinetiQ, said Unmanned Aerial Vehicles (UAVs) offer advantages over traditional aircraft and even satellites.
"The principal advantage is persistence - that you would be there all the time," he told BBC News. "A satellite goes over the same part of the Earth twice a day - and one of those is at night - so it's only really getting a snapshot of activity. Zephyr would be watching all day."
The latest flight was conducted at the US Army's Yuma Proving Ground in Arizona.
The Zephyr flew non-stop for 82 hours, 37 minutes.
That time beats the current official world record for unmanned flight set by the US robot plane Global Hawk - of 30 hours, 24 minutes - and even Zephyr's own previous best of 54 hours achieved last year.
However, the Yuma mark remains "unofficial" because QinetiQ did not involve the FAI (Federation Aeronautique Internationale), the world air sports federation, which sanctions all record attempts.
The US Department of Defense funded the demonstration flight under its Joint Capability Technology Demonstration (JCTD) programme.
The 30kg Zephyr was guided by remote control to an operating altitude in excess of 18km (60,000ft), and then flown on autopilot and via satellite communication.
It tested a communications payload weighing approximately 2kg.
At first sight, the propeller-driven Zephyr looks to be just another model aircraft, and it is even launched by hand. But this "pilotless" vehicle with its 18-metre wingspan incorporates world-leading technologies.
Its structure uses ultra-lightweight carbon-fibre material; and the plane flies on solar power generated by amorphous silicon solar arrays no thicker than sheets of paper. These are glued over the aircraft's wings.
To get through the night, the propellers are powered from lithium-sulphur batteries which are topped up during the day.
"A lot of effort has gone into power storage and light-weighting the systems," explained Mr Kelleher. "Lithium sulphur is more than double the energy density of the best alternative technology which is lithium polymer batteries.
The engineers from the Farnborough-based company are now collaborating with the American aerospace giant Boeing on a defence project codenamed Vulture.
This would see the biggest plane in history take to the sky, powered by the sun and capable of carrying a 450-kilo (1,000lb) payload.
US commanders say the design must be able to maintain its position over a particular spot on the Earth's surface uninterrupted for five years.
QinetiQ is the public company that used to be the British Government's military research department, before they privatised it.
QinetiQ's page on Zephyr