The U.S. Air Force on Friday signed a contract to purchase two planes from Boeing that were intended for a now-defunct Russian airline and instead will be converted for the Air Force One mission.

Boeing was awarded the contract to modify its two commercial 747-8 aircraft, although a price of the contract was not given.

"Because the aircraft were purchased through commercial contracting procedures, price and other related details are commercial-competition sensitive, meaning the negotiated price paid is not releasable," the Air Force said in a statement.

Russian airline Transaero originally agreed to buy the aircraft, but the firm went bankrupt, never acquired the planes and did not complete its payments, Defense One reported.

The Air Force had planned to develop two new aircraft, but now says it will be cheaper to convert thee existing planes rather than build new ones.

"This award is a significant step towards ensuring an overall affordable program," said Darlene Costello, the principal deputy assistant secretary of the Air Force, in a statement. "As we move forward, we will continue to seek and implement cost savings opportunities."

President Trump has attacked the cost of building the aircraft from scratch.

"Boeing is building a brand new 747 Air Force One for future presidents, but costs are out of control, more than $4 billion. Cancel order!" Trump said on Twitter in December.

The Air Force said the modified aircraft will be ready for the president to use in 2024.