Updated throughout at 5:55 p.m.

President-elect Donald Trump upended years of Pentagon procurement planning with a tweet on Thursday, announcing he had asked Boeing Co. to price an upgrade of its F-18 Super Hornet jet that could replace Lockheed Martin Corp.'s F-35, the most expensive U.S. weapon system ever.

"Based on the tremendous cost and cost overruns of the Lockheed Martin F-35, I have asked Boeing to price-out a comparable F-18 Super Hornet!" Trump said Thursday in a post on Twitter.

Based on the tremendous cost and cost overruns of the Lockheed Martin F-35, I have asked Boeing to price-out a comparable F-18 Super Hornet! — Donald J. Trump (@realDonaldTrump) December 22, 2016

Lockheed's $379 billion F-35 Joint Strike Fighter, which is assembled in a massive mile-long factory in Fort Worth, is intended to be the mainstay fighter of the Air Force, Navy and Marine Corps, replacing several older planes including Boeing's F-18.

Development of the F-35 is more than a decade in the works, and Lockheed is planning to build more than 2,400 of the aircraft for the U.S. and allied air forces, a project that will create tens of thousands of jobs at factories across the country and overseas.

The exact impact of Trump's tweet wasn't immediately clear. The Pentagon has scaled back purchases of the F-18, which lacks stealth and other high-tech capabilities of the F-35, and would require extensive design changes to be comparable to the newer plane.

Lockheed shares fell 2 percent after Trump's tweet in after hours trading, while Boeing rose 0.5 percent.

The Pentagon had requested just two F-18 Super Hornets for the fiscal year that began in October. Congress has indicated in spending bills that it would like to buy at least 12 more.

Meanwhile, the F-35 program has had $81 billion in obligations for the last 10 fiscal years. Two contractors, Lockheed and United Technologies Corp., account for 97 percent of the spending, according to an analysis by Bloomberg Government. Yet the program has had repeated challenges, with defense officials saying it would need as much as $500 million extra to finish its development phase.

The fiscal 2017 defense authorization would allow an additional $9.9 billion for the F-35 this fiscal year. The total includes $3.3 billion in funding for the Navy and Marine Corps versions and modifications, and $4.8 billion for the Air Force F-35 variant and modifications.

Trump summoned the chief executives of both companies to his Mar-a-Lago resort in Florida on Wednesday, as well a group of top Pentagon officials, to discuss the costs of the F-35 program and also Boeing's proposed replacement for Air Force One, the presidential aircraft. Boeing CEO Dennis Muilenburg said he told Trump the new Air Force One would be built for less than $4 billion, less than what Trump said the plane would cost.

Lockheed CEO Marillyn Hewson said in a statement after the meeting that she had assured Trump the company would continue efforts to reduce the F-35's costs. She didn't announce any new promises.

Lockheed spokesman William Phelps said the company had no comment. Boeing didn't have an immediate comment, spokesman Todd Blecher said.

On Monday, Pentagon officials defended the F-35 fighter program and said the cost isn't spiraling out of control.

Critics of the Pentagon's most expensive weapons system ignore the progress made since the program was reset in 2011, when it was six years behind schedule and $13 billion over budget, said Lieutenant General Christopher Bogdan, who heads the office responsible for developing and acquiring the fighter jet.

"This program is not out of control," Bogdan said. "Since 2011, we have basically been on schedule. Since 2011, we have basically been on budget."

Bogdan said he estimates flight testing could end by February 2018.

The $379 billion F-35, known as the Joint Strike Fighter, is the first jet created to serve the vastly different combat missions of the Air Force, Navy and Marines, and it has been plagued by malfunctions of its cutting-edge technology. The Pentagon's top weapons tester, Michael Gilmore, has warned officials in at least four memos since August that the F-35's development should be extended past next year because of continued deficiencies to its combat systems, weapons accuracy and air-to-ground gun.

Two of the three services have declared their versions of the F-35 as having initial combat capability as development winds down and Lockheed prepares to churn out the aircraft at high rates of production. Manufacturing costs are falling and the company has said it expects to assemble the aircraft for about $85 million apiece by decade's end, comparable to the cost for current-generation fighters.

Lockheed and defense officials have been locked in negotiations over terms for the 10th and largest-yet order for the jets. The Pentagon provided the company with a $1.28 billion down payment in November to continue production while the two sides hammer out a contract valued as much as $7.19 billion for 90 aircraft.

Bloomberg