Amazon.com and Blue Origin founder Jeffrey P. Bezos speaks at the Air Force Association's Air, Space & Cyber Conference in Oxon Hill, Md., on Sept. 19. (Jim Watson/AFP/Getty Images) (JIM WATSON/AFP/Getty Images)

The United Launch Alliance, a joint venture of Lockheed Martin and Boeing, has selected Blue Origin to supply the first-stage engine for its new rocket, the companies announced Thursday.

The deal marks a significant partnership between the stalwarts in the national security launch business and Blue Origin, the upstart rocket company founded by Amazon chief Jeffrey P. Bezos. (Bezos also owns The Washington Post.)

ULA and Blue Origin originally announced the partnership in 2014, but Blue Origin’s BE-4 engine was still in development and ULA kept another engine, the AR1, built by Aerojet Rocketdyne, as a potential backup.

The contract is a significant victory for Blue Origin, the little-known space company Bezos founded in 2000. It opens up a new line of business for the company, which is developing its own rocket, called New Glenn, that would be powered by the BE-4 engines. Blue Origin also intends to compete for national security launches — and could end up bidding against ULA.

“We are pleased to enter into this partnership with Blue Origin and look forward to a successful first flight of our next-generation launch vehicle,” Tory Bruno, ULA’s chief executive, said in a statement.

For years, ULA had a monopoly on national security launch contracts, and it was criticized for using a Russian-made engine in its Atlas V rocket. As tensions between Russia and the United States grew, a top Russian official threatened to cut off supply of the engines for use in Pentagon launches. The company is now developing a new rocket, called the Vulcan Centaur, that would be powered by the BE-4.

Bruno said that rocket “is making strong progress in development and is on track for its initial flight in mid-2020.”

Blue Origin has been developing its BE-4 engine for several years. It uses liquefied natural gas and liquid oxygen as propellant and has 550,000 pounds of thrust at sea level. Unlike the AR-1 engine, which was funded in part by the government, the BE-4 “requires NO taxpayer dollars,” Blue Origin says on its website.

In its statement, ULA did not say how many engines it planned to procure or how much money was involved in the deal, which was first reported by the Wall Street Journal. A spokeswoman said details of the contract are proprietary.

Steven Warren, a spokesman for Aerojet Rocketdyne, said the company would continue to develop its AR1 engine and that it was confident in its progress and performance. The company was previously selected to supply the upper-stage engine for ULA’s Vulcan rocket.

“We’re still building this engine,” Warren said of the AR1. “It’s the ideal engine for all sorts of solutions.”

Asked whether the company had a customer for it, he said, “We’re working on that.”

Phil Larson, assistant dean at the College of Engineering and Applied Science at the University of Colorado at Boulder, said the partnership of an upstart such as Blue Origin with a stalwart such as ULA “benefits both companies.”

“The fact that we’re in this new era in space calls for new kinds of partnerships,” he said.