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.