Stratolaunch -- which is designed to release rockets that will carry satellites into space -- has a 385-foot wingspan, features six engines used by the Boeing 747, stands 50 feet tall and can carry more than 500,000 pounds of payload.
And it has those 28 wheels.
The twin-fuselage aircraft, the baby of Microsoft co-founder Paul Allen, was pulled out of its Mojave Air and Space Port hangar in California to begin fueling tests -- the first of many ground tests.
Jean Floyd, Stratolaunch's chief executive officer, said the goal is to have a launch demonstration as early as 2019.
"Over the coming weeks and months, we'll be actively conducting ground and flight line testing at the Mojave Air and Space Port," Floyd said in a statement. "This is a first-of-its-kind aircraft, so we're going to be diligent throughout testing and continue to prioritize the safety of our pilots, crew and staff."
Stratolaunch will take off from a runway. It will fly to the approximate cruising altitude of a commercial airliner before launching a launch vehicle. "As the launch vehicle rockets into orbit, Stratolaunch will fly back to a runway landing for reloading, refueling and reuse," Allen says.
Allen's Vulcan Aerospace has worked with aerospace pioneer Burt Rutan's company, Scaled Composites, on the ambitious project.
The duo collaborated on the launch of SpaceShipOne in 2004.
Allen isn't the only US billionaire with the dreams and pockets big enough to take on space.
Like Musk, Bezos wants to reuse rockets to drive down the cost of space travel. But for the time being, Blue Origin is mostly interested in conducting suborbital flights for tourists.
CNN's Matt McFarland, Thom Patterson and Jackie Wattles