Potential orders involve about 20 each of the planemakers' most popular families of jets -- the Airbus A320 or Boeing 737 in the medium-haul, narrow-body class and the Airbus A330 or Boeing 777 in the long-range, wide-body category, the people said.
None of the parties involved agreed to comment.
The sources confirmed a Bloomberg News report that Delta Air Lines Inc. (DAL) was considering buying planes including 10 to 20 of the A330 or the 777 wide-body aircraft worth $4.3 billion.
Delta told an investor conference on March 4 that it needed to evaluate its needs for wide-body jets, but played down the possibility of a large order because of recent investments in its relatively young fleet.
The airline said it saw "opportunities in the marketplace" to add selectively to its wide-body fleet and would be talking, for example, to Airbus. Airlines typically engage both major aircraft makers in any discussion to seek the best prices.
Delta already operates all four of the aircraft types involved in the talks at both ends of the spectrum, making it possible for the plane-makers to offer aircraft without having to shoulder the heavy costs of helping the airline switch suppliers.
The talks come on the heels of an exceptionally busy two weeks for aircraft orders as airlines chase fuel savings while trying to grow or replace their fleets. Industry body IATA earlier edged up its forecast for airline profits this year.
On Tuesday, budget Irish airline Ryanair handed Boeing Co. (BA) its largest European order ever, a deal for 175 Boeing 737 jets worth $16 billion at list prices. The deal came a day after Indonesia's Lion Air on Monday picked European rival Airbus for a $24 billion order.
Lion Air had been an exclusively Boeing customer for jets.