In 2018, the United States Air Force (USAF) and Boeing discussed the F-15X or Advanced F-15, a proposed single-seat variant based on the F-15QA to replace USAF F-15C/Ds. Improvements included the AMBER weapons rack to carry up to 22 air-to-air missiles, infrared search and track, advanced avionics and electronic warfare equipment, AESA radar, and revised structure with a service life of 20,000 hours. Single and two-seat variants were proposed, called F-15CX and F-15EX respectively, with identical capabilities. The USAF opted for the two-seat variant, which can be flown either by a single pilot or by a pilot and WSO for complex missions and, someday, controlling collaborative combat aircraft. One reason for this decision is that only two–seat F-15 models remained in production.

The USAF bought the F-15EX to maintain fleet size as F-22 production ended, the F-35 was delayed, and its F-15Cs aged. Although it is not expected to survive against modern air defenses by 2028, the F-15EX can perform homeland and airbase defense, enforce no-fly zones against limited air defenses, and deploy standoff munitions. In July 2020, the U.S. Defense Department ordered eight F-15EXs over three years for $1.2 billion. In August 2020, the USAF announced plans to replace F-15Cs in the Florida and Oregon Air National Guards with F-15EXs. The F-15EX made its maiden flight on 2 February 2021. The first F-15EX was delivered to the USAF in March 2021, and was flown to Eglin Air Force Base in Florida for further testing.

On 7 April 2021, its official name Eagle II was announced. The FY2021 defense appropriations bill funded F-15EX procurement at $1.23 billion for 12 aircraft, bringing total orders to 20 aircraft. By May 2022, the USAF had ordered 144 F-15EXs. It has proposed reducing its orders to 80. The first operational F-15EXs are not to receive conformal fuel tanks. The Air Force's proposed budget for fiscal 2024 includes funds to buy 24 more F-15EXs, which would bring the planned fleet up to 104 aircraft.