Two of the Pentagon's specially reinforced "Doomsday" planes, designed to withstand the heat from a nuclear blast, were grounded after being damaged by a tornado, the Air Force said Friday. The E4-B Boeing 747s, built in the 1970s during the Cold War, are essentially flying command centers that can refuel in the sky and are designed to remain airborne for days on end in times of crisis. The Air Force has four E4-Bs, which also shuttle the secretary of defense around the world. Two were damaged June 16 at Offutt Air Force Base, Nebraska when a tornado whipped through the...