WASHINGTON — The United Launch Alliance has selected Blue Origin to provide the engine for its Vulcan Centaur boost-phase launch vehicle, a big win for the Jeff Bezos-founded company.

The BE-4 design was selected over Aerojet Rocketdyne’s AR1 system, ULA announced Thursday. Two of the engines, each capable of 550,000 pounds of thrust, will power the boost vehicle.

“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 CEO, said in a statement (ULA is a joint venture of Lockheed Martin and Boeing).

“We are very glad to have our BE-4 engine selected by United Launch Alliance. United Launch Alliance is the premier launch service provider for national security missions, and we’re thrilled to be part of their team and that mission,” Blue Origin CEO Bob Smith said in the same statement. “We can’t thank Tory Bruno and the entire United Launch Alliance team enough for entrusting our engine to powering the Vulcan rocket’s first stage.”

The selection comes four years and 10 days after a Sept. 17, 2014, news conference in Washington where Bezos and Bruno unveiled the BE-4.

The selection of the engine is a major point in Vulcan’s somewhat dramatic history.

For years, ULA had a monopoly on military space launch capabilities, with the company’s Atlas V launch vehicle relying on Russian made RD-180 engines to get to orbit. That agreement worked until allegations came to light of Russia invading Ukraine in 2014, setting off a wave of questions on Capitol Hill as to why the U.S. was relying on Russian technology to get key military systems into space — and lining the pockets of Russian elites with close ties to President Vladimir Putin in the process.

The ensuing fight, spurred in part by ULA’s rival SpaceX, ended with Congress banning long-term use of the Russian engines and with ULA deciding to design a new launch vehicle — the Vulcan, unveiled in April 2015.

Aerojet Rocketdyne — which in 2015 considered a bid to buy ULA outright for a reported $2 billion, before being rejected by Boeing and Lockheed — isn’t locked out entirely on the program, however, as the company’s RL10 engine had been previously selected for the Centaur’s upper stage.

After the announcement, Aerojet CEO Eileen Drake tweeted: “We’re excited to be part of the #Vulcan team, with our RL10C-X engine powering the upper-stage. The RL10 has been the nation’s premier high performance upper-stage engine for decades.”

ULA says Vulcan is expected to be ready for its first flight in mid-2020. Other component manufacturers include Northrop Grumman, L‑3 Avionics Systems and RUAG.