Singapore Airlines has become the first carrier to commit to the proposed stretched Boeing 787-10X, with an order for 30.
The carrier disclosed the agreement - which is conditional on a formal launch - as it firmed orders for 30 more Airbus A350-900s.
Delivery of the 787-10X would take place in the 2018-19 financial year. The carrier has not made an engine selection for the Boeing twinjets.
"It is conditional upon Boeing formally launching the 787-10X programme," the airline says. It values the combined aircraft agreement at $17 billion.
Delivery of the A350s would take place in 2016-17. The carrier has placed options on a further 20 of the type, which could be converted to the larger -1000. A350s are only available with Rolls-Royce Trent XWB engines.
"Today's aircraft orders are among the biggest in Singapore Airlines' history, helping to ensure that we retain our industry leading position," says chief executive Goh Choon Phong.
"These new aircraft will provide opportunities to grow and renew our fleet and enhance our network, benefiting customers by offering more travel options and the latest in-flight cabin products."