Boeing is “rapidly moving towards the launch” of the 777X, Vice President and General Manager-Airplane Development Scott Fancher said on the sidelines of the Paris air show. The two versions of the aircraft are far advanced in the development process and the company is promising its customers an improvement in fuel efficiency of at least 20% compared to current models.

Boeing, which launched the 787-10 at Le Bourget, is understood to be several months away from making the 777X plans firm. Several airlines, including British Airways, Emirates Airline, Lufthansa and Qatar Airways have publicly expressed strong interest in the aircraft.

Air Lease Corp. Chairman and CEO Steven Udvar-Hazy at the air show said he sees more need for “design refinements” before he would consider a launch order. Fancher instead insists the 777X is “very firm in the configuration, the design is very mature.”

Boeing is marketing the -8X as a 777-300ER replacement, in addition to the longer 9-X. Both aircraft will share an almost identical wing. “The wing unlocks the efficiency of the airplane,” Fancher said, and he believes the “sweet spot” of the market will actually move from 300-350 seats to the roughly 400 seats the 9X will accommodate.

The 777-8X is planned to offer a range of up to 9,400 nm while the longer version will fly up to aroumd 8,000 nm, the same as the 777-300ER. Fancher does not see range as being the subject of a “big debate”. Some airlines, including Lufthansa’s CEO Christoph Franz have complained that Boeing and Airbus tend to design their new widebodies around the greater range requirements of Middle East carriers.

Boeing may consider freighter versions of the 777X and the 787 at some point in the future, but if and when that happens “will be determined by the customers.” Fancher had “no comment” when asked about a potential 777-10X stretch of the -9X.

The company is not hearing any complaints from customers about the folding wing concept planned for the proposed 777X family. “I think it will be readily accepted,” Fancher said. Boeing has had “almost no questions about it” recently, although there were a lot of discussions earlier in the development phase.

Boeing plans to fold the outer part of the wing upwards to be able to better accommodate the aircraft at space-constrained airports, although the company does not reveal how long the wings are actually going to be.

The company initially will use the fourth iteration of the composite wings introduced for the 787 program.

Fancher told Aviation Week the folding arrangement will only be for the tip of the wing. “It is as complex as a landing gear door.” Because it will be so far outboard, only “a couple of wires” will be affected crossing the fold. Boeing once studied a similar concept for the 777-300ER, but eventually dropped it. Fancher stressed the earlier design would have affected many more systems and the control surfaces, unlike the design for the 777X.