Even before Hurricane production began in March 1937, Sydney Camm had embarked on designing its successor. Two preliminary designs were similar and were larger than the Hurricane. These later became known as the "N" and "R" (from the initial of the engine manufacturers), because they were designed for the newly developed Napier Sabre and Rolls-Royce Vulture engines respectively. Both engines used 24 cylinders and were designed for over ; the difference between the two was primarily in the arrangement of the cylinders – an H-block in the Sabre and an X-block in the Vulture. Hawker submitted these preliminary designs in July 1937 but were advised to wait until a formal specification for a new fighter was issued.