Microsoft co-founder Paul Allen has spent the last six years working on a giant aircraft capable of launching rockets to space. Today, his company Stratolaunch Systems literally rolled that plane out of its hangar in the Mojave Desert for the first time ever.

It’s called the Stratolaunch aircraft, and it’s massive. The plane has a 385-foot wingspan, which makes it the largest in the world by that metric. It weighs about 500,000 pounds dry, but that can swell to a maximum takeoff weight of 1.3 million pounds. Stratolaunch moves all that weight across the ground on 28 wheels, and eventually will carry its cargo through the air thanks to six 747 aircraft engines.

Paving a new highway to space through the sky

Why so big? Allen plans to use the Stratolaunch plane as an airborne rocket launcher. Instead of taking off from a launchpad, which requires lots of fuel, Stratolaunch will give rockets a head start by first carrying them up into the sky. It’s already got one customer, too — private spaceflight company Orbital ATK inked a deal with Stratolaunch Systems last October to use the giant plane as a launcher for its Pegasus XL rocket, which is used to send small satellites into space.

Here's video of the first roll out of the world's largest plane. pic.twitter.com/RJarl0dFRJ — Stratolaunch (@Stratolaunch) June 1, 2017

The idea is similar to Allen’s first space venture, SpaceShipOne. That aircraft won Allen and his partners the Ansari X Prize in 2004, a competition started by the X Prize Foundation to create the first private reusable spacecraft. SpaceShipOne also went on to serve as the basis for the spacecraft used by Richard Branson’s spaceflight company Virgin Galactic. (Virgin Galactic also recently spun off its own air-launcher project into a company called Virgin Orbit, which uses a single 747 aircraft to launch rockets to space.)

But before Stratolaunch takes Orbital’s (or anyone else’s) rockets for a ride, it has to finish testing the aircraft. Today was just about removing the plane’s support scaffolding, letting its wheels experience the full weight of the craft for the first time, and getting it out the door. “This was a crucial step in preparing the aircraft for ground testing, engine runs, taxi tests, and ultimately first flight,” the company wrote on its website. If all that goes well, Stratolaunch could perform its first demonstration flight in 2019.