Boeing has nailed down the basic design of the 777-9 airliner, the first of the two variants planned for the new 777X series. The program remains on track to deliver the first aircraft to one of the six airlines considered launch customers in 2020, the manufacturer said August 27.

Boeing said it has reached the “firm configuration” milestone of the 777-9 effort, marking the completion of technical trade studies into the basic design of the 400-to-425-seat variant. The program can now begin detailed design of parts, assemblies and other systems for the aircraft. Once the detailed designs are completed and released, production can begin.

“The team has followed a disciplined process to complete all of the requirements for this stage of the program,” said Bob Feldmann, Boeing’s 777X program vice president and general manager. “We have an airplane and production system that are on track and on schedule, and we remain laser focused on meeting our commitments to our customers.”

To date, Boeing has received 306 firm orders and 14 commitments for 777-9/8s from Lufthansa, All Nippon Airways, Cathay Pacific, Emirates, Etihad and Qatar Airways. Dubai-based Emirates, with 150 777X orders, is the largest current customer.

The program’s production phase is scheduled to begin in 2017. List price of the 777-9 is $400 million. Boeing lists the 350-to-375-passenger 777-8 at $371 million.