The building, adjacent to Boeing's wide-body aircraft plant in Everett, Washington, covers 27 acres (11 hectares) putting under one roof the equivalent of 25 football fields, Boeing said.
It will house three large autoclaves, or pressurized ovens, for curing the jet's carbon-composite wings, which replace the conventional aluminum construction process used for the current 777.
The autoclaves are 120 feet (37 metres) long and 28 feet (9 metres) in diameter and weigh more than 1 million pounds (453,600 kg) each, large enough to hold two 737 fuselages, Boeing said.
Boeing broke ground for the plant in August 2014, saying at the time it expected to occupy the plant this year. As many as 1,700 contractors were employed on the project at its peak, Boeing said.
The company did not say how many people will work at the plant.