The aircraft is central to the late Paul Allen's vision for Stratolaunch when he started the company in 2011. In many ways, it's a larger parallel to Virgin Galactic's White Knight Two. It's meant to reduce the costs of spaceflight by ferrying rockets to an altitude of 35,000 feet and making it easier to complete the journey into orbit.

There have been more than a few hurdles to clear along the way. There were plans to test-fly the aircraft back in 2016, but it clearly wasn't ready. It didn't start engine tests until 2017, and taxi tests have been underway in the months since. Now, though, Stratolaunch can focus on preparing its vehicle for its core mission.

The main challenge at this stage may be landing customers. Northrop Grumman has already committed to launching its Pegasus XL rockets using the aircraft, but it's not clear who else will line up. Stratolaunch may have to work hard to woo customers, especially as it's becoming easier to launch some payloads using conventional rockets.