After years of development and months of suspense, Amazon billionaire Jeff Bezos’ Blue Origin space venture has won a high-stakes race to provide United Launch Alliance with its BE-4 rocket engine.

First word of the win came unofficially in a Wall Street Journal report, and was confirmed hours later by United Launch Alliance, a Boeing-Lockheed joint venture. United Launch Alliance CEO Tory Bruno confirmed that BE-4 engines would be used on the first stage of ULA’s yet-to-be-built Vulcan rocket.

“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,” Bruno said in a news release.

Blue Origin CEO Bob Smith said he was “very glad” to see the BE-4 engine selected as the Vulcan rocket’s prime propulsion system.

“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,” said Smith, who took on the CEO role for Bezos’ venture last year. “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.”

Bezos himself chimed in on Twitter:

Big win for @BlueOrigin today! And a big thank you to @ulalaunch for choosing our BE-4 engine. Excited to be a part of Vulcan’s mission to space. #21stCenturyEngine #AllAmericanEngine #GradatimFerociter https://t.co/K0Szt3jesi — Jeff Bezos (@JeffBezos) September 27, 2018

The announcement caps what United Launch Alliance CEO Tory Bruno has said will be a multibillion-dollar partnership on the Vulcan project. The rocket is designed to send up to 56,000 pounds of payload to low Earth orbit.

Using Blue Origin’s BE-4, a 550,000-pound-thrust engine that’s fueled by liquefied natural gas, would follow through on Congress’ requirement that ULA stop using Russian-made RD-180 engines.

Bezos has been covering the bulk of the development cost for the BE-4, which is also destined to be used on Blue Origin’s reusable, orbital-class New Glenn rocket. This month, Bezos said he’d be spending “just over a billion dollars” on the New Glenn program the next year.

New Glenn could eventually loom as a competitor for the Vulcan. That scenario was reportedly one of the concerns that emerged during months of negotiations that also focused on price and technical capabilities.

ULA’s “Plan B” was to turn to Aerojet Rocketdyne’s kerosene-fueled AR1 engine, but that engine was thought to be far behind the BE-4 on its development timeline. In recent weeks it became increasingly clear that Blue Origin held the edge, but there’s a consolation prize: Aerojet’s RL10 engine is to be used on the Vulcan’s Centaur upper stage.

The BE-4 is currently built at Blue Origin’s headquarters in Kent, Wash., and for more than a year the engines have been undergoing testing at Blue Origin’s facilities in West Texas. The current schedule calls for their use on New Glenn and ULA Vulcan flights in 2020, but that schedule could slip.

Deals already have been struck for New Glenn satellite launches in the 2020s.

Blue Origin has said that winning ULA’s nod would open the way for the development of a $200 million engine factory in Huntsville, Ala.

The company has moved into a $200 million New Glenn rocket manufacturing facility near Cape Canaveral, Fla., and aims to send up its New Glenn rockets from Launch Complex 36 nearby.

Blue Origin has a separate suborbital space program, known as New Shepard, and is currently conducting uncrewed flight tests of that spaceship in West Texas. There’s also a lunar lander program known as Blue Moon that could eventually win NASA’s support for missions in the 2020s.

Update for 2:05 p.m. PT Sept. 27: This story has been updated with the official announcement from United Launch Alliance.