The Air Force has awarded Boeing a $600 million modification to a previous contract to buy two commercial 747 jetliners, and convert them for the Air Force One.

The additional money, according to an Air Force statement, is for design upgrades to the pair of planes that were originally destined to be sold to a now-bankrupt Russian airliner, which never took delivery. The contract is for the design of modifications to ensure the planes can accommodate the communications and security needs of the president.

The actual work on the aircraft won't begin until sometime next summer, and will require a future contract, and many more millions of dollars.

The total cost of acquiring and outfitting the two planes is not known, in part because the initial purchase price of the two commercial 747-8 aircraft was not disclosed by the Air Force, which presumably purchased the previously-sold planes at a discount.

President Trump criticized the program in December, when he was president-elect, for the projected cost of designing two aircraft from scratch. After Trump met with Boeing leadership, the company said it was lowering the price, and it was later revealed that the company would sell the two existing planes to the Air Force.

The $600 million will cover the costs of redesigning the interior and avionics of the jetliners to convert them into "presidential mission-ready aircraft" by upgrading communications and electrical power systems, and adding a medical facility, an executive interior, and a self-defense system, according to an Air Force press release.

The Air Force says the design process will also look for ways to cut the final cost of the aircraft without degrading mission capabilities.

"This contract award is the next major step forward toward ensuring an overall affordable program," said Maj. Gen. Duke Richardson, the Presidential Airlift Recapitalization program executive officer, in the statement.

One cost-cutting decision has already been made. Unlike the older 747-200 presidential aircraft, the new Air Force One planes will not have the capability of mid-air refueling. So in the event of a national emergency, the president's plane would eventually have to find a safe place to land.

Pentagon officials say they cannot remember anytime when the current Air Force One aircraft ever required aerial refueling.

The decision not to retrofit the planes purchased from Boeing's commercial inventory for mid-air refueling was one of a number of cost-cutting measures made in consultation with the White House, Air Force officials said.

"As a result of these reviews, the program has simplified or eliminated a number of requirements, to include reducing the fleet from three to two aircraft, maximizing the use of commercial interiors and avionics, eliminating the aerial refueling requirement, and accepting the commercial 747-8 cooling capacity," one official told the Washington Examiner.