"We are airborne," the company said in a statement. "Currently performing various flight test maneuvers."
The aircraft will complete its test flight at about 2 p.m. Eastern Time (18:00 GMT)T. It is Stratolaunch's second test flight since April 2019.
Stratolaunch has already partnered with aerospace research development company Calspan Systems to build and test a reusable hypersonic prototype vehicle called Talon-A.
Stratolaunch's aircraft is the largest aircraft in the world. It has six Boeing 747 engines that allow the aircraft to carry a maximum payload weight of at least 500,000 pounds. The aircraft's wingspan stretches over 100 yards.