located in the south of England, which made them vulnerable to enemy air attack, the British government in 1935 commenced the "Shadow Factory" programme. These massive new "Shadow Factories", which it was intended would employ the latest production techniques, were to be set up in areas away from the immediate threat of attack and where it was felt there would be sufficient transferable skills which could be employed on the manufacturing process. For the Spitfire, Lord Nuffield offered to build the massive Castle Bromwich Shadow Factory near Birmingham. Work began on its construction on 12 July 1938. However continual problems beset the factory and despite it having been planned that it would be producing 60 aircraft per week by April 1940 it was not until June 1940 (well into the Battle of Britain) that the problems were resolved by new management, with the first 10 Spitfires being delivered that month. Thereafter production increased to reach a maximum rate of 320 aircraft per month. By the end of the war it had produced a total of 12,129 aircraft (which was over half of all Spitfires made). Heavy bomber In response to Air Ministry specification B.12/36, which was issued in July 1936 to all the major aircraft manufacturers for a four-engined heavy bomber, Mitchell designed the Type 316 which carried its bombs in both the fuselage and wings. After submitting the Supermarine proposal, Mitchell in characteristic fashion began a total redesign called the Type 317 with new wings and different gun turrets. Two prototypes of the Type 317 were ordered in March 1937. Following the death of Mitchell in June 1937 after a long battle with cancer and with Supermarine having problems meeting the demand for the Spitfire the Air Ministry realised that work on the prototypes would be delayed. Therefore, as a precaution they provided funding in 1937 for the development of the competing design from Short Brothers. This design eventually entered service as the Short Stirling. The Type 317 prototypes were destroyed by an air raid in 1940, before work had progressed to a point where they flew. Death of Mitchell Following the death of Mitchell, his deputy Harold Payn was appointed Chief Designer. However a security check in September 1939 identified that Payn had a German born wife. Concerns about the risk this posed to a major war program saw Payn dismissed. Joe Smith was promoted from Chief Draughtsman,