The first parts are going together for Boeing's very first 777X airliner in Everett.

Assembly of the wing spar, the strong beam-like structures that the wing skins are attached to, are being put together in the Spar Assembly Building.

Boeing celebrated the milestone on Monday with employees.

The 777X with composite wings made primarily of carbon fiber and resin was at risk of moving to another state. In 2014, Boeing decided to keep the 777 assembly line in Everett after Washington beat out competing states by offering $8.7 billion in tax breaks.

The Machinists Union also approved a new extended contract to keep the work after a contentious fight, which changed provisions of the contract that many saw as concessions.

Boeing says it's made a $1 billion investment in Everett, which includes a new composite wing center.

The first 777X will be assembled over much of 2018 with the first test flight in 2019, followed by what Boeing plans as the first delivery to an airline in 2020. Boeing executives say the program is on schedule.