WASHINGTON — The B-21 Raider has begun carrying out taxi tests at Air Force Plant 42 in Palmdale, California, a key step for the Northrop Grumman-made stealth bomber before it can carry out its first flight.

A U.S. Air Force spokesperson confirmed the first B-21 bomber is now taxing along the ground, but said no other details on its testing were available.

Confirmation of the B-21′s taxi tests came after a photograph of the bomber, taken from the rear and apparently showing it moving along a taxiway, emerged on social media.

“Rigorous testing is a critical step in the B-21 flight test program,” the Air Force said. “Extensive testing evaluates systems, components and functionalities. This testing allows us to mitigate risks, optimize design, and enhance operational effectiveness.”

The Air Force and Northrop Grumman plan for the initial B-21 bomber to have its first flight by the end of the year, which would be about a year after its heavily publicized rollout in December 2022. The two are also carrying out a series of ground tests to pave the way for that flight.

Air Force Secretary Frank Kendall did say at the Air and Space Forces Association’s conference in September that the first flight should happen this year “absent any unexpected surprises,” but cautioned “surprises do happen in acquisition programs.”

Officials said at the conference that engine runs — or tests the bomber’s propulsion systems — had begun. Other preparations included activating its systems, troubleshooting its fuel systems, and ensuring its doors, landing gears and control actuation systems work properly.

The B-21 will be put through a series of further flight tests after it arrives at Edwards Air Force Base in California.

The Air Force now has at least six B-21s in various stages of production and testing.