The mega jet carried out its maiden voyage over the Mojave desert on Saturday.
It is designed to carry into space, and drop, a rocket that would, in turn, ignite to deploy satellites.
It is supposed to provide a more flexible way to deploy satellites than vertical takeoff rockets because this way all you need is a long runway for takeoff.
Specifically, the wing span is 117 meters; that of an Airbus A380 is just under 80. The plane flew Saturday for about two and a half hours, Stratolaunch said. Until now, it had just carried out tests on the ground.
It hit a top speed of 304 kilometers per hour (189 mph) and reached an altitude of 17,000 feet, or 5,182 meters.
"Today's flight furthers our mission to provide a flexible alternative to ground-launched systems," he added.
But Allen died in October of last year so the future of the company is uncertain.