LONDON (Reuters) - The head of Boeing's ( BA ) defense unit said Sunday the planemaker is still "fighting through challenges" in building two delayed U.S. presidential aircraft known as Air Force One.

In 2018, Boeing ( BA ) received a $3.9 billion contract to build two 747-8 aircraft for use as Air Force One, to be delivered by December 2024, but they are now delayed until at least 2027 and 2028.

Ted Colbert, who heads Boeing Defense, Space & Security, said the planemaker is facing supply chain, inflation, workforce and other challenges in building the airplanes. Boeing ( BA ) is heavily modifying a pair of 747s for the project.

The company has lost more than $2 billion on the program. "Our team is fighting through a very, very challenging program - two very complex airplanes," Colbert said. "We've done a ton of investment in our workforce and training, efficiency, work on the factory floor."

The Boeing 747-8s are designed to be an airborne White House able to fly in worst-case security scenarios, such as nuclear war, and are modified with military avionics, advanced communications and a self-defense system.

Last year, the Biden administration opted to modify, but continue with, an Air Force One paint scheme that closely resembles the current white with two shades of blue, which dates back to President John F. Kennedy's administration, reversing a decision made by then President Donald Trump.

In December 2016, Trump extracted a promise from Boeing's ( BA ) CEO at the time, Dennis Muilenburg, that the cost of replacing Air Force One would not exceed $4 billion. Boeing's ( BA ) current CEO Dave Calhoun said later "critics were right" about the contract, saying it was priced too low.

In 2022, the U.S. Government Accountability Office (GAO) said the Air Force One program risked further delay, citing a tight labor market for mechanics, lower-than-expected security clearance rates and Boeing's ( BA ) need to switch to an alternative supplier for some interior work.