Development of the 777X, which includes the 777-9 and smaller 777-8, comes as Boeing speeds up commercial aircraft production to more than 76 a month by 2020 from 62 now, taking output to the highest level in the company's 100-year history.
Boeing expects to build the first 777-9, to be its largest twin-engine plane, in 2017. Output of the current 777 will not slow during the transition, the company has said, although some analysts remain skeptical.
The 777X, a successor to the 777 wide-body jet introduced in 1995, will have carbon-fiber composite wings with folding wingtips and an aluminum fuselage. It will use 12 percent less fuel and be 10 percent cheaper to operate than competing jets, Boeing said.
The 777X will compete with Airbus' A350, which also competes with the Boeing's 787 Dreamliner.
Boeing's announcement on Thursday that the 777-9 had reached "firm configuration" means the plane's basic design and capabilities have been determined, setting the stage for design of specific parts and systems.
The company's stock rose 2 percent to $131.87 on the New York Stock Exchange, about in line with the broader market.
The 777-9 will carry 400-to-425 passengers and have a range of 7,600 nautical miles (14,075 kilometers), making it the largest plane in Boeing's lineup by seat capacity. It will be the largest twin-engine plane in the world, though smaller than the four-engine Airbus A380.
Under specifications Boeing released earlier this month, its biggest plane, the 747-8, is no longer its largest in terms of standard seat capacity.
"The program is right where we want it to be," said Bob Feldman, who heads the 777X program. "We have an airplane and a production system that are on track and on schedule."
Boeing is using automated technology to help assemble the 777X, and installing autoclaves, or industrial ovens, in a massive new building in Everett, Washington, to cure the wings under heat and pressure, next door to the assembly line.
Boeing said the 777X wingspan will be 235 feet, 5 inches (71.8 meters) with wingtips extended and 212 feet, 8 inches (64.8 meters) with wingtips folded.
The 777X has garnered 306 firm orders and another 14 commitments.
(Alwyn Scott - Reuters)