By Katherine FauldersPresident-elect Donald Trump tweeted today that the government should cancel its order with Boeing for a new Air Force One, citing costs of the project."Boeing is building a brand new 747 Air Force One for future presidents, but costs are out of control, more than $4 billion. Cancel order!" Trump said in his tweet.Trump then made a surprise appearance, emerging from the elevators at Trump Tower in New York City before scheduled meetings and commented on the project."Well, the plane is totally out of control. It's going to be over $4 billion. It's for Air Force One program, and I think it's ridiculous," Trump said. "I think Boeing is doing a little bit of a number. We want Boeing to make a lot of money, but not that much money."The Air Force announced in January 2015 that the Boeing 747-8 aircraft has been chosen as the upgrade for Air Force One from the current VC-25 model, which is also manufactured by Boeing."The presidential aircraft is one of the most visible symbols of the United States of America and the office of the president of the United States," Secretary of the Air Force Deborah Lee James said in a statement at the time."The Boeing 747-8 is the only aircraft manufactured in the United States when fully missionized meets the necessary capabilities established to execute the presidential support mission while reflecting the office of the president of the United States of America consistent with the national public interest," she added.Boeing said in a statement today that the company is "currently under contract for $170 million to help determine the capabilities of these complex military aircraft that serve the unique requirements of the president of the United States.""We look forward to working with the U.S. Air Force on subsequent phases of the program allowing us to deliver the best planes for the president at the best value for the American taxpayer," the company added.The main differences between Air Force One and a regular 747 are state-of-the-art navigation, electronic and communications equipment, as well as the interior, which includes offices, a conference/dining room and living space for the president and first lady, according to Boeing's website.Boeing has long been mulling a retirement of the 747 line. Air Force One is filled with communications gear and defensive gear and costs much more than a regular 747 aircraft.