"During final load testing on the 777X static test airplane, the team encountered an issue that required suspension of the test," a Boeing spokesman said in a statement.
A report by the Seattle Times said a cargo door exploded outward during a high-pressure stress test on the ground. These "final load" tests, which are designed to push the aircraft to the breaking point estimated by the manufacturers, are part of the aircraft certification process and are overseen by inspectors from the Federal Aviation Administration.
The 777X was originally scheduled to take off on its first flight this summer. That date had been postponed until early 2020 by Boeing due to problems with its General Electric engine. The plane is meant to replace the 777 and can carry 400 to 425 passengers.
The suspension of the testing for the 777X comes as Boeing is completing changes required by regulators on its 737 MAX, which has been grounded worldwide after two crashes resulted in 346 deaths.