Boeing officially began assembling the second KC-46A Tanker last week as workers loaded the aircraft’s wing spar into an Automated Spar Assembly Tool.

The on-time milestone brings the program another step closer to delivering the U.S. Air Force’s next-generation aerial refueling tanker.

The event comes less than two months after the KC-46 program began to assemble the first of four Engineering, Manufacturing and Development test aircraft. The wing spar, the main structural component of the wing, is 82 feet 5 inches long and provides critical support for flight loads and the weight of the wings.

“Completing production of the four test aircraft on schedule is our priority as we prepare to enter the flight test phase of the program. The program continues to meet or exceed all contract milestones,” said Major General John Thompson, U.S. Air Force Program Executive Officer for Tankers.

The first test aircraft is expected to roll out of the factory in January 2014, while the second is scheduled to leave the factory in March. Boeing plans to fly the fully provisioned tanker for the first time in early 2015 and make the first delivery in 2016. The company expects to build and deliver the first 18 KC-46As by 2017 and a total of 179 by 2027 if all options under the contract are exercised.

“We’re exactly where we want to be right now. One year from now, all four test aircraft will be out of the factory, and the first two will be flying,” said Maureen Dougherty, Boeing Vice President and KC-46 Tanker Program Manager.

The KC-46A is based on the Boeing 767 commercial jetliner, a proven airframe in service as an airliner, freighter and tanker. Boeing has delivered more than 1,050 767s worldwide.

“Production of the first tanker is going smoothly and remains on schedule,” said Scott Campbell, 767 Vice President and General Manager, Boeing Commercial Airplanes.