United Airlines was putting the finishing touches on Monday (June 28) to an order potentially worth tens of billions of dollars.
Industry sources says it's for 270 narrow body jets, in a bid to secure a post-crisis recovery at favourable prices.
The order could include up to 200 Boeing 737 MAX planes and 70 Airbus A321neos.
A deal would be worth around $33 billion at the most recently published list prices.
But analysts say airlines typically pay less than half price for deals of this size.
None of the parties commented ahead of an announcement expected at a United investor event on Tuesday (June 29).
Sources warned that negotiations are complex and numbers of units can shift.
The deal would be the industry's biggest since the aviation sector was hammered by travel restrictions.
It would also accelerate a recovery for the MAX, which has been rebuilding momentum after being rocked by a safety crisis.
Analysts though say it would not resolve a strategic stalemate which has seen Airbus dominate the busy market for larger single-aisle jets.
United is considered one of the industry's most influential buyers, whose whopping purchases can set the tone for decades.