The mega jet carried out its maiden voyage over the Mojave desert.
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.
It was built by an engineering company called Scaled Composites.
The aircraft is so big its wing span is longer than a football field, or about 1.5 times that of an Airbus A380.
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.
"What a fantastic first flight," said Jean Floyd, CEO of Stratolaunch.
"Today's flight furthers our mission to provide a flexible alternative to ground launched systems," he added.
Stratolaunch was financed by Paul Allen, a co-founder of Microsoft as a way to get into the market for launching small satellites.
But Allen died in October of last year so the future of the company is uncertain.
