The Pentagon told the congressional auditors that “ongoing design issues, modification rework, and workforce challenges slowed the progress toward modifying two Boeing 747-8 aircraft into presidential aircraft,” a report released Wednesday said.

The United States, under President Donald Trump, signed a $3.9 billion contract with Boeing in 2018 to have two planes designed, modified, and tested to serve as Air Force One by 2024, but persistent delays have pushed the project past its delivery date.

In order for the aircraft to meet Air Force One specifications, Boeing needs to heavily modify the jets to meet travel and top-level security needs, which has slowed down the production process considerably. Trump has dropped a requirement for the VC-25B planes to have air-to-air flying capabilities, which would allow them to receive fuel from a tanker aircraft while in flight.

Progress on the aircraft has stalled as a result of issues around decompression and the environmental control system design, and Boeing’s incomplete certification plans and aircraft design have also pushed back the timetable for testing, the GAO said. The aircraft manufacturer has also been unable to keep a steady workforce for the project.

“Boeing still faces challenges hiring and retaining qualified mechanics due to ongoing market conditions, according to VC-25B officials,” the report said. “Program officials said that approval rates for mechanics to acquire necessary clearances remain a workforce limitation.”

Last fall, Boeing announced plans to lay off 10% of its workforce, despite industrywide talent shortages.

As Boeing tries to push through its scheduling woes, Trump has instead procured a $400 million Boeing 474 from Qatar after threatening in February to find alternatives to the VC-25B project. The gifted Qatari jet has not only raised ethical and security concerns, but it will likely cost taxpayers $1 billion to service and update the jet to Air Force One standards.

The 10-digit chunk of change is on top of the projected development and procurement cost of the two Boeing aircraft, according to GAO, which is nearly $6.2 billion—at least $2 billion more expensive than the cost originally outlined in the contract.

Boeing and the Pentagon did not respond to Fortune’s requests for comment.

Years of delays

Though Trump commissioned Boeing for the new jets during his first term, there’s a good chance the president will likely not fly on the new Air Force One during his administration. In February 2024, Boeing revised its delivery schedule from May 2027 to December 2029, but a Boeing senior official told Reuters in February the program could stretch “years beyond” 2029. 

The issues plaguing Boeing’s production now look similar to the delaying variables from years past. In a June 2022 GAO report, the auditors said Boeing’s aircraft mechanic workforce was limited because of a competitive labor market and that many of the skilled workers needed on the project were unable to get security-clearance approval.

“Employees must meet stringent security requirements to work on the VC-25B program because of its presidential mission,” the report said. “VC-25B officials said that Boeing continues to work with the program office to improve the prescreening process for applicants to ensure timely processing of security clearances.”

Earlier in Trump’s second term, Boeing sought help from Elon Musk, then leader of the Department of Government Efficiency (DOGE). Boeing CEO Kelly Ortberg said at a Barclays conference in February the advisory has helped remove some production bottlenecks. Musk visited Boeing’s San Antonio facility in December 2024.

“The president’s clearly not happy with the delivery timing. I think he’s made that well known,” Ortberg said. “And Elon Musk is actually helping us a lot.”