The twin-fuselage plane, called the Stratolaunch, flew for about 2 1/2 hours over the Mojave Desert in Southern California, reaching a speed of 189 mph and an altitude of 17,000 feet, the Stratolaunch team said.
Its wingspan is longer than a football field.
The company seeks to cash in on higher demand in coming years for vessels that can put satellites in orbit, competing in the United States with other space entrepreneurs and industry stalwarts such as Elon Musk's SpaceX and United Launch Alliance - a partnership between Boeing and Lockheed Martin.
A giant six-engine aircraft with the world's longest wingspan completed what company officials called a superb initial flight over California's Mojave Desert, bringing to life a dream held by the late Microsoft co-founder Paul G. Allen.
Though Stratolaunch is the largest plane by wingspan, another craft, the helium-filled Airlander 10, takes the title of the longest aircraft now flying with a length of 302 feet (92 m).
However, neither Krevor nor Jean Floyd, the chief executive of Stratolaunch, said anything about the test flight program, including when the plane will fly again and how long the overall test program will last. The company has already signed at least one customer in Orbital ATK, which plans to use Stratolaunch to send its Pegasus XL rocket into space. Allen died in October 2018 while suffering from non-Hodgkins' lymphoma, just months after the plane's development was unveiled.
In August, Stratolaunch said it was developing a "family of launch vehicles" that could be deployed from the plane to carry satellites. Stratolaunch, which was founded by Allen, is vying to be a contender in the market for air-launching small satellites.
Even so, the flight Saturday was billed as a milestone and an engineering marvel. He did not specify what those things were, and briefing participants did not take questions.
"The airplane very nicely, smoothly rotated and really just lifted off the ground", he said. "It flew very much like we had simulated and like we predicted".
The idea is to fly the plane to 10 km (6.2 miles) before releasing satellites into orbit.