From the Document: "In FY2020, the Trump Administration proposed to buy eight F-15EX aircraft for $1.1 billion, the first installment toward a planned initial buy of 144 planes. Subsequently, Congress authorized $985.5 million and appropriated $1.05 billion for the program. The Administration's FY2021 budget requests $1.27 billion for 12 more F-15EXs; the plan set forth in FY2020 had called for 18 in FY2021. This acquisition represents a change from previous Air Force plans to procure only stealthy 'fifth-generation' fighter aircraft. What is an F-15EX, and why might the Air Force have changed plans?"
CRS In Focus, IF11521
Congressional Research Service: https://crsreports.congress.gov/