The U.S. Air Force’s top officer wants the service to develop an affordable, lightweight fighter to replace hundreds of Cold War-vintage F-16s and complement a small fleet of sophisticated—but costly and unreliable—stealth fighters. The result would be a high-low mix of expensive “fifth-generation” F-22s and F-35s and inexpensive “fifth-generation-minus” jets, explained Air Force Chief of Staff Gen. Charles Brown Jr. If that plan sounds familiar, it’s because the Air Force a generation ago launched development of an affordable, lightweight fighter to replace hundreds of Cold War-vintage F-16s and complement a small future fleet of sophisticated—but costly and unreliable—stealth fighters. But...