Boeing [BA] has  achieved a milestone by starting production of the KC-46A aerial refueling tanker, according to a company statement.

Boeing workers in Everett, Wash., yesterday loaded the first wing spar, the main structural component of the wing. Checking in at 82 feet, five inches, the spar provides critical support for flight loads and the weight of wings when the plane is not flying.

The wing spar load for Boeing’s first KC-46A tanker. Photo: Boeing.

The KC-46A’s next major contractual milestone, the Air Force’s critical design review (CDR), starts in July.

The tanker is based on Boeing’s commercial 767-200ER, a proven airframe in service as an airliner, freighter and tanker. Boeing employees are also preparing the 767 production line for the assembly of the next-generation tanker’s aft and forward body structures. The aircraft will be assembled in November and roll out of the factory in January.

Boeing, in June 2014, will begin installation of military-unique systems on the aircraft at Boeing Field in Seattle as well as testing. First flight for the fully provisioned tanker is scheduled for early 2015 with first delivery in 2016.

Boeing expects to build and deliver the first 18 KC-46As by 2017 and a total of 179 by 2027 if all options are exercised under the contract.