Six years after Microsoft co-founder Paul Allen started up Stratolaunch Systems, the billionaire’s air-launch venture brought its humongous twin-fuselage airplane out in the open for the first time today.

“Stratolaunch came out of the hangar for fuel testing,” Allen said in a tweet that featured an aerial photo of the plane. More pictures were posted to Stratolaunch’s website.

Stratolaunch CEO Jean Floyd said the plane’s emergence from its hangar at California’s Mojave Air and Space Port was part of a “major milestone in its journey toward providing convenient, reliable and routine access to low Earth orbit.”

The plane is designed to carry up to three Orbital ATK Pegasus XL rockets at a time into the air, and then set them loose to launch payloads into orbit.

Today’s outing signaled that the aircraft’s initial construction phase is complete, and that it’s ready to start ground and flightline testing. Two tugs pulled the plane out onto the tarmac in front of the hangar’s roughly 400-foot-wide doors.

Over the next few days, each of the plane’s six fuel tanks will be filled independently to make sure that the tanks are properly sealed and that the fueling mechanisms work, Stratolaunch said.

Then the tanks will be drained, and the aircraft will be returned to its hangar for weight and balance testing. Over the months ahead, the plane’s systems will undergo further checks in preparation for taxi tests, flight operations and airworthiness testing.

Stratolaunch says the first launch demonstration, involving a single Pegasus XL, will take place as early as 2019.

The plane, built in partnership with Mojave-based Scaled Composites, is mostly made of carbon composite. Key elements of the craft, including its six engines and eight sets of landing gear, were taken from Boeing 747 jets.

The Stratolaunch plane’s 385-foot wingspan makes it the world’s widest airplane.

Now that the plane is capable of resting “weight on wheels,” Stratolaunch was able to weigh it for the first time, Floyd said in today’s update. As expected, the weight came in at about 500,000 pounds. It’s designed to carry 250,000 pounds of fuel, and payloads of up to 550,000 pounds. All that adds up to a maximum takeoff weight of 1.3 million pounds.

The advantage to Stratolaunch’s air-launch system is that multiple satellites could be sent into different types of orbits during a single sortie – with little advance notice, from any airport with a long enough runway, and with the capability to fly around bad weather.

Floyd said the Pegasus XL is just one of the options for launch.

“We’re actively exploring a broad spectrum of launch vehicles that will enable us to provide more flexibility to customers,” he said.