The KC-46A, a derivative of the company's 767 commercial jetliner, is slated to replace the Air Force's aging KC-135 fleet, and Boeing is under contract to deliver initial tankers by 2017.
"All four test aircraft are moving through production to support our transition to ground and flight testing later this year," said Maureen Dougherty, Boeing vice president and KC-46 Tanker program manager. "Our joint Boeing and U.S. Air Force team continues to deliver on our commitments to the warfighter."
The first flight of an engineering and manufacturing development KC-46 test aircraft -- without its aerial refueling systems -- will take place this year. Boeing said delivery of the first production aircraft is scheduled for early 2016.
Boeing plans to produce a total of 179 of the tankers for the Air Force by 2027.
NBC reportedly holds celebs hostage to Jimmy Fallon's show
Aaron Carter is still in love with Hilary Duff