The step comes just weeks before Air Force officials are expected to sign off on the tanker's final design, the next major milestone as Boeing gears up to deliver the first KC-46A aerial-refueling tanker in 2016.
"The Air Force is really excited and pleased that our No. 1 modernization priority has begun fabrication and entered the factory at Everett," Maj. Gen. John Thompson, U.S. Air Force program executive officer for tankers, said in a statement.
Over the next 14 years, Boeing is to deliver 179 tankers to the Air Force, having won the contract in 2011. The KC-46 is based on the commercial 767-200 Extended Range jet, also assembled in Everett.
On Wednesday, Boeing workers slid the wing spar, the main structural component of the wing, into a jig on the 767 production line. The spar is 82 feet, 5 inches long. Company employees also are preparing the line for assembly of the KC-46 tanker's aft and forward body structures.
Boeing plans to roll this first KC-46 out of the factory in January. Employees at Boeing Field will install military systems on the aircraft next June, with the first flight of the completed airplane set for early 2015.
"We're proud to support the U.S. Air Force with a production line that emphasizes quality, efficiency and safety," Scott Campbell, general manager of the 767 Program, said in a statement.
Boeing will provide 18 tankers to the Air Force by 2017 under a fixed-cost contract. The company would deliver 179 by 2027 if the Air Force exercises all options, a step that will largely be based on how well Boeing performs during the initial stage of the contract.
The Air Force sought for nearly a decade, beginning in 2001, to replace its fleet of KC-135 tankers, which are 50 years old on average. Boeing beat out EADS, the European parent of Airbus, in 2011 for the contract, which is worth as much as $35 billion.
An Airbus executive recently suggested Boeing is struggling with weight problems with the tanker, an issue that could give EADS another chance at supplying the Air Force with tankers. Boeing said the tanker remains on track.
For Boeing, the tanker contract means decades of work, keeping alive an ailing 767 line. The commercial 767 had just 59 orders remaining in May.
The tanker contract also positions Boeing for additional sales internationally. The company anticipates selling 100 to 200 tankers outside the U.S, a Boeing executive told Reuters earlier this month.