Derived from the General Electric GE90 with a larger fan, advanced materials like ceramic matrix composites (CMCs), and higher bypass and compression ratios, it was designed to improve fuel efficiency by 10% compared to the GE90.

In February 2012, GE announced studies on a more efficient derivative of the GE90, calling it the GE9X, to power both the -8 and -9 variants of the new Boeing 777X.

A design change required for the GEnx was the addition of bypass doors between the booster and high-pressure compressor which open into the airflow path to reduce the chance of ice crystals entering the core.

The tip clearance at the front of the HP compressor was modified as a result of early running experience.

SETT testing started on May 16, 2017, at Peebles, Ohio, 13 months after FETT; it was the first engine built to the finalized production standard for certification.

[10] During the FAA 150 hr block test, the variable stator vane (VSV) lever arm failed and its redesign led to a 3–month delay.

[10] Suspended on a 19 ft (580 cm) strut, the fourth engine of the program began flight testing at the end of 2017.

[18] In February 2018, the GE9X's first flight was delayed by problems discovered in the high-pressure compressor (HPC) variable stator vanes (VSV) lever arms.

Also a routine A Check on the 747 testbed CF6 engines discovered fan-case corrosion and high pressure turbine airfoils on allowable limits.

[12] A second phase, of 18 flights, began on December 10 to evaluate the engine control software and hot-and-high performance and lasted until the first quarter of 2019 before FAA certification the same year.

[21] At the end of January, the turbine case and rear frame strut were damaged during the blade out test and relevant components were redesigned.

In early May, the flight test program was completed after 320 hours run time, during which high-altitude cruise fuel burn was established.

Engines were modified to a final certifiable configuration standard before the maiden flight of the 777X, delayed beyond the previously expected June 26 by a stator problem at the front of the 11-stage high-pressure compressor.

On September 28, GE announced its FAA type certificate, as eight test engines completed 8,000 cycles and 5,000 hours of running.

[1] Its 61:1 overall pressure ratio should help provide a 5% lower thrust specific fuel consumption (TSFC) than the XWB-97 with maintenance costs comparable to the GE90-115B.

[5] Fourth generation carbon fiber composite materials, comprising the bulk of the fan blades, make them lighter, thinner, stronger, and more efficient.

CMCs are not used for the first-stage turbine blades, which have to endure high centrifugal forces and extreme heat.

The low-pressure turbine airfoils made of titanium aluminide (TiAl) are stronger, lighter, and more durable than nickel-based parts.