While working at this company, Northrop focused on the flying wing design, which he was convinced was the next major step in aircraft design. His first project, a reduced-scale version tested in 1940, ultimately became the giant Northrop YB-35. The Northrop XP-56 Black Bullet, a welded magnesium fighter[6] was one of the more significant of his World War II designs, along with the Northrop P-61 Black Widow, the first American night interceptor, of which more than 700 were constructed.[7]