The Department of the Air Force on Friday said it awarded Sierra Nevada Corp. a $13 billion contract to replace the service’s aging E-4B Nightwatch “doomsday planes” that would fly during a nuclear war. The company will develop and produce the Survivable Airborne Operations Center, the name for the aircraft that will succeed the E-4B, and is expected to finish the work by July 10, 2036. The Air Force is obligating $59 million in research, development, test and evaluation funds to Sierra Nevada