United Airlines has firmed its order for 25 Airbus A350-900s, taking the order backlog for the twin-jet family to 530 aircraft.
Deliveries of the A350s will start in 2016 and continue to 2019.
United's formalisation of its commitment, which it originally disclosed at the end of last year, follows a similar deal by the airline to take 25 Boeing 787s.
All A350s are currently only available with Rolls-Royce Trent XWB engines.
"With this order we are investing in United's future, gaining a widebody aircraft that will help us reduce operating costs while still providing state-of-the-art comfort to our customers," says United president John Tague.
As with the 787 order, the A350 pact includes options for another 50 aircraft.
"It's a true testament to what the A350 XWB has to offer that one of the largest airlines in the world has selected it to be an instrumental part of its future," says Airbus chief operating officer for customers John Leahy.