Customers, including the 777’s biggest buyer, Emirates Airline, have pushed for the 777X as early as possible. But Boeing has been slow to formally launch the program with what is known as “authority to offer.”

To be sure, the timing could be affected by the 787. Tinseth said Boeing’s main focus is on getting its 787 Dreamliner flying again after the plane was grounded worldwide because batteries burned on two of the jets last month.

But if the 777X is offered this year, a firm industrial launch could follow eight to 12 months after Boeing starts taking orders, Aboulafia said, and “entry into service by the end of the decade would be quite feasible.”

It was not immediately clear how airlines, which tend to shy away from complexity, would respond to the proposed folding wingtips if retained in the final design. But Boeing is expected to woo them with increased range and capacity made possible by design changes including the brand-new and longer carbon-composite wings. The average passenger load of the most popular version could increase to slightly over 400 seats from 365 seats, according to industry sources briefed on the plans.

Airbus is marketing a mini-jumbo, 350-seat version of its mainly composite A350 to try to dent the success of the 777, and is likely to argue that 777 features such as fold-back wingtips could add maintenance risk.