Northrop Grumman Corp. NOC said late Tuesday that it started engine runs as part of the testing of its B-21 Raider “stealth” bomber. The program’s ground testing continues to “progress,” the defense giant said. The engine runs are at company facilities in Palmdale, Calif. “Engine testing is an essential milestone for the program as the world’s first sixth-generation aircraft continues on the path to flight test,” the company said. “The B-21’s first flight will remain a data driven event that is monitored by Northrop Grumman and the United States Air Force,” it said. Shares of Northrop Grumman dropped 2% in the extended session Tuesday after ending the regular trading day down 0.2%.

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