The US Air Force awarded Boeing a contract worth $24.1 million for additional design work on Air Force One.

The design work was not specified in the contract notice the Air Force posted online 10 April. Work will be performed in Seattle, Washington, and is expected to be complete by December 2018.

The design work is “to advance the maturity of the air vehicle design beyond the preliminary design level on the VC-25B,” the Air Force wrote in its notice, using the aircraft type name. “This modification supplements work already taking place under the PAR contract, including the acquisition of two commercial Boeing 747-8 aircraft and VC-25B preliminary design activity.”

The USAF awarded Boeing a $600 million contract to begin preliminary design work for Air Force One in September 2017. The aircraft modifications include adding a mission communication system, electrical power upgrades, a medical facility, executive interior, self-defense system and autonomous ground operations capabilities onto two Boeing 747-8s.

The USAF plans to replace its two 747-200-based Air Force One aircraft delivered in 1991 with two 747-8s originally built for Russian carrier Transaero, which filed for bankruptcy in 2015 before it could take delivery.

President Trump has been critical of Air Force One’s cost and personally pushed Boeing for cost reductions in 2017. After negotiations, he claimed repurposing commercial airliners, as well as other cost-savings measures promised by Boeing, would save the government $1.4 billion.

The new presidential aircraft are scheduled to begin flying in 2024. The total cost of the programme is $3.9 billion, according to the White House.