F-35 Joint Strike Fighter: Actions Needed to Address Manufacturing and Modernization Risks, Report to Congressional Committees [open pdf - 2MB]
From the Document: "The acquisition cost for the F-35 program increased substantially in 2019, partially due to the program's addition of estimated costs for modernization of hardware and software systems, referred to as its Block 4 efforts. This is the fifth report under the provision that Congress included in statute for GAO [Government Accountability Office] to review the F-35 program annually until the program reaches full-rate production. This is also the first report under another provision in statute to review the program's production and Block 4 progress annually through 2024. Among other objectives, this report assesses (1) the program's production performance and (2) the program's modernization cost estimate and development progress. GAO reviewed Department of Defense (DOD) and contractor documentation and interviewed DOD officials and contractor representatives."
Government Accountability Office: https://www.gao.gov/