The KC-46 aircraft, which refuels other aircraft mid-air, has been plagued with problems, including foreign object debris found in the planes and issues with the mechanism for delivering fuel.
The cargo lock flaw was discovered during a test flight carrying cargo when some locks started to disengage, raising concerns cargo could shift and create a hazardous condition.
Boeing said it has so far fixed the problem on four jets. The testing and certification of the new design "closes out the flight restrictions and allows the Air Force to carry cargo and passengers on its missions," Boeing said in a statement.The Air Force plans to buy 179 of the tankers. Boeing landed the $49 billion contract in 2011 to build the tankers based on its 767 commercial jets.