Northrop Grumman Corp.’s stock fell by 6% Thursday after the defense contractor posted a fourth-quarter loss on costs associated with the production launch of its B-21 stealth bomber. The company also signaled that it could face cost overruns on the bomber, which is able to fly without human pilots onboard. Northrop NOC, -5.91% and other defense contractors are facing higher costs and a reduced chance of receiving inflation payments from the U.S. government. Northrop said it lost $535 million, or$3.54 a sh ...