The downgrading of plans for F-35 upgrades, revealed in GAO’s report “F-35 Program: Actions Needed to Address Late Deliveries and Improve Future Development,” is the latest challenge for the vital — as well as troubled and expensive — program.

“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.”

Block 4 is meant to boost the Lockheed Martin-made jet’s weapons capabilities, sensors and sensor fusion, and comes on the heels of another upgrade known as Technology Refresh 3, or TR-3. However, Block 4 is now at least $6 billion over budget and years behind schedule, in part due to delays with TR-3.

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.

Program officials then decided to focus on Block 4 capabilities that could be delivered by 2031 at the earliest — still at least six years behind schedule — and that wouldn’t require the additional power and cooling capacity of a planned engine upgrade. Block 4 is still on track to deliver improved electronic warfare, weapons, communication and navigation capabilities, GAO said, but a more detailed list was not available to auditors. Some capabilities, including those that depend on the improved engine, will be delayed, and others struck entirely because they are no longer needed, the report said.

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.

Those costs are also $89.5 billion more than the rebaselining that was done in 2012. The F-35’s lifetime cost, including sustainment costs, is more than $2 trillion.

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.

The contract marks the largest deal in the history of the company’s Missiles and Fire Control unit.

The agreement covers the fiscal years 2024 through 2026 and calls for the procurement of 1,970 Patriot Advanced Capability-3 Missile Segment Enhancement interceptors and associated hardware for the U.S. and allies, according to the service’s statement.

“PAC-3 MSE’s recent combat performance solidified it as a must-have capability for America and its allies around the world,” Jason Reynolds, Lockheed’s vice president for Integrated Air and Missile Defense, said in a company statement. “With this contract, we’ll be delivering record numbers of PAC-3 MSE for years to come.”

The contract underscores Washington’s push to replenish missile stockpiles and strengthen supply chains as conflicts in Ukraine and the Middle East heighten the demand. The PAC-3 MSE, which uses hit-to-kill technology, is designed to counter tactical ballistic missiles, cruise missiles, hypersonic threats and aircraft.

Seventeen partner nations, including the U.S., field PAC-3 missiles.

“The muti-year buy enables “the Army to procure a larger quantity of missiles for more rapid delivery, thus filling our inventory faster,” Maj. Gen. Frank Lozano, who leads the Army’s Program Executive Office Missiles and Space, said in the service’s statement.

Lockheed said it plans to deliver more than 600 interceptors in 2025, ramping up production nearly two years ahead of the award thanks to internal investments.

The contract “reflects disciplined acquisition and responsible stewardship of taxpayer resources,” Joseph Giunta, the Army’s senior contracting official at Redstone Arsenal, Alabama, said in the statement.

Lockheed Martin told Defense News earlier this summer it was working toward delivering 650 PAC-3 MSE missiles per year by 2027. It is currently producing roughly 550 each year.

The company is also looking at how it might possibly ramp up to 650 missiles a year earlier than planned, Reynolds told Defense News in July at the Association of the U.S. Army’s LandEuro conference in Wiesbaden, Germany.

“What we’re endeavoring to do in partnership with the U.S. government, is to pull that left as much as we can,” he said.

The company is also looking at “the efficiencies and streamlining and doing everything we can to stretch those dollars actually to take that to a higher capacity, upwards of around 750 per year by 2027,” Reynolds said.

The government has not yet officially released the number for a top production level, but “if you take that trajectory and you project it forward into the future years, you will see as well north of 1,000 and then even much higher after that,” he said.

The report by the Vienna-based International Atomic Energy Agency said that as of June 13, Iran had 440.9 kilograms (972 pounds) of uranium enriched up to 60%, an increase of 32.3 kilograms (71.2 pounds) since the IAEA’s last report in May.

The report stated that this figure is “based on the information provided by Iran, agency verification activities between 17 May 2025 and 12 June 2025 (the day preceding the start of the military attacks), and estimates based on the past operation of the relevant facilities.”

That material is a short, technical step away from weapons-grade levels of 90%.

The confidential report also said Iran and the IAEA have not reached an agreement on resuming inspections of sites affected by Israeli and U.S. bombing in June.

The only site inspected since the war has been the Bushehr Nuclear Power Plant, which operates with Russian technical assistance.

The director general of the U.N. nuclear watchdog, Rafael Grossi, said that “technical modalities to enable the full resumption of Agency inspection should be concluded without delay,” the report stated.

The report stated that while the withdrawal of UN inspectors from Iran during the war “was necessary given the overall security situation, Tehran’s “subsequent decision to cut cooperation with the IAEA was ”deeply regrettable.”

As of June 13, Iran’s total enriched uranium stockpile was 9874.9 kilograms, which represents an increase of 627.3 kilograms since the last repot in May, the report said.

The IAEA said that since June 13, it has “not been able to conduct the in-field activities required to collect and verify Iran’s declarations used to estimate the changes to the previously reported stockpile.”

According to the IAEA, approximately 42 kilograms of 60% enriched uranium is theoretically enough to produce one atomic bomb, if enriched further to 90%.

The IAEA also reported that inspectors have not been able to verify Iran’s near bomb-grade stockpile for over two and a half months, which it called “a matter of serious concern.”