The Air Force Monday awarded a contract modification to Boeing [BA] to build the fourth lot of KC-46 next-generation tanker aircraft.

The contract modification, worth $2.9 billion, includes 18 additional aircraft, data, two spare engines, five wing refueling pod kits, initial spares, and support equipment being produced under the basic contract.

A KC-46 tanker refuels an F/A-18 fighter during testing. (Boeing photo)
A KC-46 tanker refuels an F/A-18 fighter during testing. (Boeing photo)

Boeing received its first two production lots, for 7 and 12 aircraft respectively, in August 2016. The third lot for 15 aircraft was awarded in January 2017.

“We’re excited to partner with the Air Force on an aircraft that will provide its fleet unmatched capabilities and versatility,” said Mike Gibbons, Boeing KC-46A tanker vice president and program manager in a statement Monday. “This is another big milestone for the team and we look forward to delivering this next-generation, multi-role tanker for years to come.”

Boeing is now on contract to build and deliver 52 aircraft. To date, the 34 platforms awarded in previous lots have all been built and are in various stages of completion, Chick Ramey, a company spokesman, said in August. The company has completed all flight tests required before initial delivery, and is on schedule to deliver the first aircraft this October, he added.

The Air Force has expressed a desire to purchase 179 aircraft total to replace its fleet of aging KC-135 Stratotankers, originally built by Boeing.