“The F-35 remains critical to our national defense, as well as that of our partners and allies, and is expected to retain critical roles for decades to come,” the report said. “After nearly 20 years of aircraft production, however, the F-35 program continues to overpromise and underdeliver.”

The F-35 program originally aimed to have Block 4’s 66 capabilities — which later swelled by more than a dozen — completed by 2026, but that deadline first slipped to 2029 and then further. A 2024 Defense Department review found the program was not on pace to start delivering the bulk of Block 4 upgrades until the mid-2030s, partly due to technical challenges, according to GAO.

GAO said program officials acknowledged the new plan for Block 4 doesn’t meet its original intent, but that the revised version could be realistically achieved within new cost, schedule and performance goals, and allow capabilities to be delivered at a more predictable pace.

A 2021 estimate also found Block 4 costs had grown from the original $10.6 billion to $16.5 billion; an updated estimate won’t be ready until the end of 2025.

In a statement to Defense News, Lockheed Martin stood by its work on the F-35 and pledged to keep moving forward on Block 4.

“The F-35 is combat proven, offers the most advanced capability and technology, and is the most affordable option to ensure America and its allies remain ahead of emerging threats,” Lockheed Martin said. “In partnership with the F-35 Joint Program Office, we will deliver 170 [to] 190 F-35s this year and continue fielding Block 4 capabilities to ensure the F-35 maintains its unmatched dominance in the skies.”

The GAO report also raised other concerns about the F-35 program, including rising costs and ongoing production delays.

The F-35’s acquisition costs — which include development and procurement — have grown to more than $485 billion, as of December 2023. That’s a nearly 10% increase from the December 2022 estimate of $442 billion, and more than double its original baseline cost from 2001.

The F-35’s engine will also have to work harder to produce the level of power and cooling capacity the jet’s Block 4 upgrades will require, GAO said. This will result in more wear and tear on the engine, accounting for $38 billion of the program’s lifecycle cost estimate growth.

Lockheed is also still behind schedule on the TR-3 hardware and software that will make Block 4 possible. The company now expects to begin delivering those TR-3 elements in 2026, about three years after its original goal. Those delays were caused by problems maturing its integrated core processor and stabilizing its software so that TR-3 reliably starts up.

The TR-3 delays in 2023 led to the Pentagon ordering a halt, which ultimately lasted a year, to F-35 deliveries. Lockheed ended up storing dozens of new jets, which were slated to have TR-3, at locations such as its Fort Worth, Texas, facility as it scrambled to get TR-3 working.

Ultimately, Lockheed managed to get an interim version of the software working well enough for the Pentagon to begin accepting the jets, even though they were unable to fly in combat. This was done in part to avoid the risk of having more than 100 high-value F-35s parked together.

However, the TR-3 problem has worsened Lockheed’s years-long issue with delivery delays. In 2024, all 110 F-35s the company delivered were delivered late, with an average delay of 238 days behind schedule. That is far below where the company was in 2021, when it delivered 22 of its 142 aircraft late, with an average delay of 16 days behind schedule.

As of May, Lockheed was still working to deliver 20 remaining aircraft that were meant to be delivered in 2024.

Parts shortages have also contributed to F-35 delays, GAO said. This February, program officials said, Lockheed had 52 in-the-works jets that had to be taken off the production line and temporarily stored because necessary parts were so late.

That month, there were more than 4,000 parts shortages in the final stage of the production line, twice the historic average, according to the Defense Contract Management Agency. More than 1,600 of those parts are required for TR-3 hardware, the report said. And the flap on the front of the F-35’s wings is one of the main parts driving production delays over the last two years, GAO said.

DCMA officials told auditors that Lockheed has plans it hopes will fix the parts shortages, such as working with suppliers to more quickly identify risks to the supply chain. However, even with those plans, GAO said, parts delays and shortages will continue to delay deliveries through the rest of 2025.

GAO also noted that even when Lockheed delivers aircraft up to 60 days late, it still is able to receive partial incentive fees, which can total hundreds of millions of dollars. Auditors advised the Pentagon to rethink its use of those fees to avoid rewarding late deliveries.

RTX subsidiary Pratt & Whitney also failed to deliver any of the jet’s F-35 engines on time in 2023 and 2024, the report said. And last year, GAO said, those engines were 155 days late on average.

DCMA has repeatedly urged Pratt to speed up its engine deliveries, so far without success, GAO said. However, those engine delays haven’t affected aircraft production, auditors said.

The F-35 Joint Program Office and Pratt & Whitney did not respond to requests for comment by the time of this story’s publication.