-- President-elect Donald Trump triggered another media firestorm after he tweeted on Tuesday that "costs are out of control" on Boeing's Air Force One project and calling for the contract with the Seattle-based company to be canceled.
"We want Boeing to make a lot of money, but not that much money," Trump told reporters later that day at Trump Tower in New York City.
According to Trump, the cost of a new Air Force One plane is $4 billion. But how true is that exactly?
A History of Boeing's Air Force One
Franklin D. Roosevelt was the first American president to fly aboard a Boeing Air Force One aircraft, and the tradition has continued for more than 70 years.
While technically any plane carrying the American president is referred to as "Air Force One," multiple presidents have traveled for the last 50-plus years on aircraft that are specifically designed to transport them across the country and around the world.
These designs include the ability to refuel in midair, on-board electronics that "are hardened to protect against an electromagnetic pulse ... and advanced secure communications equipment [which] allow the aircraft to function as a mobile command center in the event of an attack on the United States," according to the White House.
Any current Air Force One is also required to have four engines, which allows for the extra weight of the Boeing 747's sensor equipment, power units and self-defense, among other requirements necessary to carry the commander-in-chief through the skies.
The Current Fleet of Air Force One
Two aircraft currently operate as the official Air Force One jets -- both are Boeing 747-200B series and were delivered under President George H. W. Bush in 1990. By the time the current fleet retires in 2024 (the date the new fleet is expected to enter service), the aircraft will be more than 30 years old.
And while that doesn't sound like much, one Air Force official with knowledge of the matter told ABC News that keeping an aircraft beyond its 30-year cycle can mean that some of the technology aboard becomes dated and increasingly hard to maintain, and that any delay in replacing the current aircraft could lead to significant problems.
The New Contract
In January 2015, the Air Force chose the Boeing 747-800 series as the next aircraft to fly the president. Then, in May of this year, the Air Force issued a "Request for Proposals," the first step toward an eventual contract. That proposal was for two planes, but could ultimately increase to three planes during negotiations. Once Boeing responds to this proposal, the full contract will be ratified and design work will begin.
According to Boeing, the new 747-8 series will be more fuel efficient and able to carry more weight -- 157,000 more pounds, to be exact -- than its 200B series counterpart. It would also fly farther and faster than any other Air Force One jet, with the ability to travel 1,000 more nautical miles and at a top speed of Mach 0.855, making it the fastest presidential jet ever.
Breaking Down the Cost
The U.S. government has currently spent $170 million on the project, according to Boeing, despite the $4 billion that has been reported by the Trump team. The Air Force said it has awarded three contracts to Boeing this year totaling $170 million to help bring down design costs for the future program.
In February, the Air Force projected a five-year estimated total cost of the Air Force One program of $2.778 billion, according to budget documents released at the time. While this is only for the cost of research, testing and development of the future Air Force One fleet, additional costs would likely be incurred for the actual purchase of the aircraft.
However, the Air Force confirmed to ABC News it intends to pay for the new aircraft out of the research money.
The five-year breakdown for the program includes:
$351.2 million in Fiscal Year 2017$625.6 million in Fiscal Year 2018$741 million in Fiscal Year 2019$573.7 million in Fiscal Year 2020$487.3 million in Fiscal Year 2021
So Where Does the $4 Billion Figure Come From?
According to Vice President-elect Mike Pence, Trump found the $4 billion number from a Government Accountability Office report. And while published in March of this year, the report estimated a total cost for research and procurement of two presidential aircraft at $3.2 billion.
Pentagon spokesman Capt. Jeff David told ABC News that while that may sound like a lot of money, taxpayers have to remember this is a "system of systems."
"It’s multiple aircraft. And it’s not all Boeing. This a system that is going to have many different companies that are providing the systems that go on it," David said.
"We simply don’t know the exact figure. I know there are figures being thrown around, but we still have to get to the point of understanding fully what the requirement is before we can put a price tag on it," David added.
ABC News' David Kerley, Devin Dwyer and Katherine Faulders contributed to this report.