Zephyr took off from the Yuma Proving Ground at 1440 BST (0640 local time) on Friday, 9 July.
After only 31 hours in the air, it had bettered the official world record for a long-duration flight by a drone; but then it kept on going, unencumbered by the need to take on the liquid fuel that sustains traditional aircraft.
Clear skies at 60,000ft delivered copious amounts of sunshine to its amorphous silicon solar arrays, charging its lithium-sulphur batteries and keeping its two propellers turning.
At night, Zephyr lost some altitude but the energy stored in the batteries was more than sufficient to maintain the plane in the air.
Zephyr is set to be credited with a new world endurance record (336 hours, 24 minutes) for an unmanned, un-refuelled aircraft - provided a representative of the world air sports federation, who was present at Yuma, is satisfied its rules have been followed properly.
Its fortnight in the sky easily beats the 30 hours, 24 minutes, set by Northrop Grumman's RQ-4A Global Hawk in 2001.