Stratolaunch system aircraft
Stratolaunch carrier aircraft completes low-speed taxi test Stratolaunch Systems Corp.

The world's biggest aircraft in terms of wingspan cleared a milestone test after firing its engines and taking a stroll on the runway.

Paul Allen's Stratolaunch Systems' massive carrier aircraft has been built for mid-air rocket launches. The plane, which boasts two fuselages and a wingspan of 385ft (enough to cover a football field), will carry the rocket up to a certain altitude before it's launched into the orbit. The effort is aimed at reducing the cost of rocket launches.

But before any of that could happen, the behemoth of a plane has to pass a number of tests. Its engines were fired a couple of months back and now the company has demonstrated its ability to steer and stop in what has been called a "low-speed taxi" test.

As seen in the video, in the latest test, the aircraft fired its six Pratt & Whitney turbofan engines and took a slow stroll down the runway under its own power. Though it did not accelerate for take-off, the test at 28mph ensured that the plane inches one step closer to the historic first flight.

Throughout the test, all major systems including steering, braking, anti-skid, and telemetry were monitored and each of them worked as per expectations.

"This was another exciting milestone for our team and the program," said George Bugg, aircraft program manager of Stratolaunch Systems.

"Our crew was able to demonstrate ground directional control with nose gear steering, and our brake systems were exercised successfully on the runway. Our first low-speed taxi test is a very important step toward first flight. We are all proud and excited."

Now, the company will continue with these tests, increasing the speed of the aircraft to prepare it for the first test flight and eventual rocket launches from the stratosphere. However, there's still no word on the timeline set for the tests and first flight.