The old rules said you had to choose: stealth or speed. The US picked stealth. Russia went fast. China is trying to throw away the rule book. Chinese researchers unveiled a development in aircraft design last month that could propel the nation’s next-generation stealth bombers into supersonic flight, ending the trade-off between speed and stealth that has troubled the American and Russian air forces for decades. Since the 1930s, scientists have been studying a type of aircraft called the flying...