The Pentagon’s efforts to modernize the F-35 Joint Strike Fighter with its Block 4 upgrade have encountered fresh obstacles, pushing the expected completion date to at least 2031, the Government Accountability Office (GAO) has reported.
The Block 4 package was meant to significantly expand the aircraft’s combat potential with cutting-edge weapons, next-generation sensors, and upgraded electronic warfare tools. Instead, the program has been plagued by rising costs, technical complications, and persistent delays. The GAO found that Block 4 will cost over $6 billion more than first projected and will be delivered years later than planned.
In response, the F-35 Joint Program Office has reorganized the Block 4 effort—together with the Technology Refresh 3 (TR-3) upgrades—into a dedicated subprogram. This restructuring, mandated by Congress in 2023, aims to improve accountability after repeated setbacks. TR-3, a critical enabler of Block 4 capabilities, has been particularly problematic, causing widespread delivery delays. In 2024, Lockheed Martin delivered 110 jets, but all were late by an average of 238 days, according to GAO findings.
Officials now plan to complete updated program documents by fall 2025, which will decide how many of the original 66 planned upgrades can still be fielded. The GAO has warned that the downsized program will continue to face cost uncertainties and performance risks until a full and accurate cost estimate is available.
