WASHINGTON — Boeing won’t be delivering the first KC-46 tanker to the Air Force in 2018 as planned, due to Defense Secretary Jim Mattis’ surprise ouster from the Pentagon, a source told Defense News on Monday.

The Air Force had intended to accept the first KC-46 by the end of December and was awaiting signature from Mattis, which would finalize the delivery plans, Reuters first reported on Dec. 20.

However, later that day, Mattis announced that he would step down from the top Defense Department post in February, and just three days later, President Donald Trump tweeted that Mattis would wrap up his work as secretary of defense by the end of 2018.

With sweeping changes in leadership at the Pentagon imminent, a decision on KC-46 was pushed out, a source close to the program told Defense News. The situation was further complicated because Deputy Secretary of Defense Patrick Shanahan — who by the end of Dec. 31 will temporarily take over Mattis’ role as defense secretary — is a former Boeing executive and must recuse himself from all decisions involving the company.

Boeing declined to confirm that the delivery would not take place on Dec. 31.

“KC-46 remains a top priority and we look forward to delivering tanker aircraft in partnership with the Air Force,” said Kelly Kaplan, a spokeswoman for the company, in a statement to Defense News.

The Air Force did not immediately respond to a request for comment.

The delay is a blow for Boeing, which had promised to deliver the first tanker by the end of 2018 after a string of missed delivery dates stemming back to August 2017.

“We continue to make steady progress toward final certification of the KC-46 tanker,” Boeing’s CEO Dennis Muilenburg said in an October earnings call. “We are working with our U.S. Air Force customer toward completing all the steps required to deliver the first tanker aircraft this quarter."

The company has now racked up more than $3 billion in pre-tax charges on the program, as it is responsible for any costs beyond the $4.9 billion fixed price contract value originally awarded in 2011.

Boeing officials are hopeful that the Air Force will be able to move forward with delivery after a set of meetings in early 2019, one source said.

But at the moment it is unclear who will authorize the KC-46 delivery with Shanahan barred from the process, though it may fall to Air Force Secretary Heather Wilson or to Ellen Lord, undersecretary of defense for acquisition and sustainment.

The Air Force plans to buy 179 KC-46s throughout its program of record. McConnell Air Force Base in Kansas is set to become the first installation to receive the new tankers, and training for pilots and boom operators has already begun.