This is what's really going on with the new Air Force One

Boeing's upcoming replacement for the US Air Force's fleet of aging presidential transports may have run into an unexpected snag.

President-elect Donald Trump tweeted Tuesday morning that he believes the Boeing 747-8-based jet - which was selected last year - is too expensive and that he intends to cancel the order.

"Boeing is building a brand new 747 Air Force One for future presidents, but costs are out of control, more than $4 billion," Trump tweeted. "Cancel order!"

However, Trump's $4 billion price tag for the Presidential Aircraft Recapitalization program - AKA the next Air Force One - seems to be a bit off. More like $800 million off. According to a report by the US Government Accountability Office published in March, the PAR program has an estimated total cost of $3.21 billion in 2016 dollars.

In a statement to Business Insider, Boeing wrote:

"We are currently under contract for $170 million to help determine the capabilities of this complex military aircraft that serves the unique requirements of the President of the United States. We look forward to working with the US Air Force on subsequent phases of the program allowing us to deliver the best plane for the president at the best value for the American taxpayer."

The current fleet of Presidential aircraft - which the Air Force calls the VC-25A- entered service in 1990, during the George H.W. Bush administration. The two planes - with tail numbers 28000 and 29000 - are highly modified examples of the Boeing 747-200B airliner.

"The PAR aircraft will be a four engine wide-body, commercial derivative aircraft, uniquely modified to provide the President, staff, and guests with safe and reliable air transportation with the equivalent level of security and communications capability available in the White House," The GAO wrote in its report.

Air Force decided to forgo an open competition for the VC-25A replacement. Opting, instead, to award the sole-source contract to Boeing on the basis that there is simply no other manufacturer in the US capable of fulfilling the requirements. In fact, the only other aircraft in the world with similar capabilities is the European Airbus A380 superjumbo.

"The Boeing 747-8 is the only aircraft manufactured in the United States (that) when fully missionized meets the necessary capabilities established to execute the presidential support mission," Air Force secretary Deborah James said in a statement at the time of the selection.

The PAR program will consistent of two 747-8-derived jets set to enter service in 2024 - which means, even if Boeing delivers on time, Trump will not have the opportunity to fly on the aircraft.

By that time, the existing Ronald Reagan-ordered VC-25A aircraft will be 34 years old.

The commercial Boeing 747-8 Intercontinental airliner retails for $367.8 million.