President Donald Trump's confirmation Sunday that the United States may accept a presidential aircraft gifted from Qatar marked the latest escalation in his clash with Boeing, which is years behind on its government contract to build the new Air Force One.

Boeing was supposed to deliver a replacement for Air Force One last year, but significant delays have cost the plane maker billions on the project. They have also drawn ire from the president, who told reporters aboard Air Force One in February that he was "not happy with Boeing" and may purchase a plane elsewhere.

Now, as the U.S. government weighs whether to use a Qatari plane as Trump's Air Force One until shortly before the end of his second term, Boeing's delays are back in the spotlight.

Here's what you need to know.

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How old is Air Force One, and what type of plane is it?

Two modified Boeing 747 planes serve as Air Force One. Both are about 35 years old and have been flying since President George H.W. Bush was in office.

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What did Boeing's Air Force One contract promise?

The Air Force awarded a contract to Boeing for two new Air Force One planes in July 2018 at a total cost of $3.9 billion, after months of negotiation.