Some six weeks after officially selecting General Electric as its “engine partner” for the 777X, Boeing has been given the green light by the its board to formally start offering the stretched twinjet to customers.

Boeing, which was originally expected to seek board approval for this milestone sometime in April, says it is “taking the next step by engaging in the marketplace, discussing more details and offering the airplane to customers – conditional on final approval to launch the program.”

In March, when it selected GE as the prime propulsion partner over competitive proposals from Rolls-Royce and Pratt & Whitney, Boeing said that the “next step for 777X may include offering the airplane to customers and eventual launch as a committed airplane program.” At the time it also laid the groundwork for the marketing and development launch of the program by appointing former 737 MAX leader Bob Feldmann as the new vice president and general manager for the 777X.

Although the company’s announcement falls short of the traditional “authority to offer” (ATO) language used in former programs, the approval is thought to be the equivalent of the traditional ATO milestone. Given the schedule of previous efforts, the May 1 board decision puts Boeing and GE on track for a potential launch around October or November 2013, and places both companies at the start of a six-year development track culminating with the entry into service of the first of two new 777 family members in 2019.

The extended twinjet series will include a 353-seat 777-8LX, sized to succeed today’s 777-300ER, and the 406-seat 777-9X, which opens up new territory in the higher capacity long-range market. The larger model is provisionally slated to debut first, with the -8LX following around 2021. As with the current longer-range 777 models which are powered exclusively by GE90-110/115Bs, the 777X will be offered only with the 100,000 lb. thrust GE9X.

The engine will be aimed at a 10% fuel burn improvement over today’s GE90-115B engines and will be configured with a 132 in. diameter composite fan case and fourth-generation composite fan blades as well as an advanced, 27:1 pressure ratio high-pressure compressor. GE is building a 90% scale GE9X high-pressure compressor development rig which will run at the company’s Oil and Gas test facility in Massa, Italy in July. The first run of the new core is expected in 2014 and the design will be frozen in 2015. The first engine will run in the 2016 timeframe, with flight testing on GE’s 747-400 flying testbed in 2017 and certification the following year.