The Mallard was powered by a single engine, but it was followed by the Cornelius XFG-1 prototypes, which were flying fuel tanks, unpowered and designed for towing by larger aircraft. These Cornelius designs were unusual for being not only forward-swept but also tailless.

In Germany, Hans Wocke was studying the problems of swept wings at the near-sonic speeds of which the new jet engines were capable. He recognised many