One day after presenting the company’s first-quarter results, Dan Burnham revealed that he will step down as CEO of Raytheon Co., effective July 1. In a statement, Burnham, 56, said, “It was a difficult decision, but I am ready to begin a new, more personal phase in my life.” Burnham joined Raytheon in July 1998 from AlliedSignal, where he was president and COO. He will be replaced by William Swanson, 54, a 31-year Raytheon veteran who’s currently president of the Lexington, Mass. company.