The Pentagon tasked Lockheed Martin Corp. with equipping a fighter jet with a missile-killing laser by 2021, a challenge that has eluded the military for over two decades.

Lockheed in November secured a $26 million deal from the Pentagon to develop a laser for a supersonic F-15 jet capable of disabling a missile or drone from a mile or more away, according to a defense department release and military officials. It is the landmark piece of a Pentagon push to develop a low-cost solution to outmatch adversaries such as China...