The Wartime Shadow Factory System
During the mid nineteen thirties a number of perceptive and well placed individuals in government, Whitehall ministries and other organisations began to recognise that another war with Germany was becoming increasingly likely. This troubling proposition brought with it numerous problems. One of these problems highlighted the situation that existing armaments suppliers would not be able to cope with the demands of a new European conflict. Materiel in the form of aircraft, aero engines, army vehicles, tanks, guns and munitions of every description would be required in vast quantities, well beyond the capacity of established armaments firms. Consequently, new solutions would have to be found to deal with the problem.
The government reasoned quite rightly that the major car manufacturers might be able to assist with the armaments deficit by harnessing their collective expertise in mass production manufacturing techniques. In 1936 the Government formed the Directorate of Aeronautical Production to administer what was eventually to develop into the shadow factory system. The general concept was that the government would finance the construction and equipping of the designated shadow factories and then turn them over to the motor industry firms to manage. Each factory built under the shadow scheme would be allotted a specific product to manufacture, with a view to replicating it precisely, and thus “shadowing” the parent company’s original design.
A large portion of the British motor industry was located in the midlands, centred on Coventry and Birmingham. One of the staunchest advocates of the shadow factory system was motor industrialist William “Billy” Rootes, chairman of the Rootes Group based in Coventry. Others sympathetic to the shadow factory cause were Maurice and Spencer Wilkes of the Rover Company. However, not all motor industry senior executives were quite so easily persuaded. Additionally, on the other side of the coin, some firms such as Rolls-Royce, who were subsequently destined to be “shadowed” particularly for Merlin aero engine manufacture, were initially not very enthusiastic about sharing their hard earned reputation for engineering excellence and technical expertise with car companies. It took all the managerial skills of Ernest Hives, later Lord Hives, to convince his Rolls-Royce board that it was fundamentally in the interests of the company, and the country as a whole, to accept the shadow factory system.
Similarly, the Engine Division of the Bristol Aeroplane Company, who had spent over a decade and a great deal of money perfecting the application of the sleeve valve in their range of radial aero engines, which included the Hercules, was very reluctant to reveal to others the technical know-how it had acquired in the manufacture of the nitrided, ground and honed thin-walled sleeve. For the sleeve valve engine to function satisfactorily, it was absolutely essential that the sleeves were manufactured from specially prepared alloy steel blanks and finished to very tight dimensional tolerances. This particular issue over the transfer of sleeve valve technology came to a head later when the Napier Company got into severe trouble with their very powerful and complex 24 cylinder sleeve valve Sabre engine, which was being manufactured in their Acton, West London works, and subsequently in a Liverpool based shadow factory. The Bristol Company was enlisted/directed by the Ministry of Aircraft Production (MAP) to assist Napier in finding a solution to the Sabre’s chronic unreliability. Eventually this was accomplished, although the Sabre still remained a complex engine to build and maintain. However, it proved to be one of the most powerful piston aero engines of the second world war, powering the Hawker Typhoon and Tempest.
It was axiomatic, if the shadow factory system was to function satisfactorily, that the “shadows” did not indulge in any unilateral developmental changes to specifications without full authorisation from the parent factory. Almost inevitably difficulties did occur when no doubt well-meaning persons, who thought they knew better, went ahead and modified components or processes, which then resulted in service failures. In the early part of the war the Castle Bromwich Aircraft factory, which had been laid down in 1938 as a shadow factory for volume Spitfire production, ran into difficulties because of the sheer scale of modifications introduced by Vickers-Supermarine to early aircraft, in order to meet ever changing operational requirements. Changes to specifications and modifications are an anathema to production engineers, and by early June 1940 not one completed Spitfire had left the Castle Bromwich factory. Inevitably, this situation could not be countenanced for very long during wartime, which eventually resulted in the removal, perhaps slightly unfairly, of the motor industry management team headed by Lord Nuffield, to be replaced by a new administration from the parent company, Vickers. For a fuller explanation of this debacle, please see the author’s paper in the WIAS series entitled, “The Castle Bromwich Aircraft Factory.”
Several piston aero engines were scheduled for shadow factory manufacture during the war. They included the Bristol Pegasus, Mercury and Hercules air cooled radial engines, together with the liquid cooled in-line Rolls-Royce Merlin and Napier Sabre. Wartime piston aero engines were by their very nature complex pieces of machinery, the Napier Sabre, as previously stated, being particularly so. Therefore, it was essential that from a pool of randomly selected components, manufactured by either parent or different shadow factories, that any given item would work satisfactorily in any given engine. This situation was achieved by the extensive use of jigs, fixtures, special tools, gauges, identical processes and the rigid adherence to specifications, employing, somewhat paradoxically, predominantly unskilled and semi-skilled labour.
In the case of the Rolls-Royce Merlin engine, this was ultimately manufactured at five major centres. Additionally, a myriad number of smaller locations were also used. These major centres were at Derby (Rolls-Royce Headquarters), Crewe, Hillington (Glasgow), Trafford Park, Manchester (the Ford Motor Company) and in the United States at the Packard Company based in Detroit. Of these the one true shadow factory, built at government expense, was Hillington. Although technically a shadow factory, this facility was actually run by Rolls-Royce management and had at its peak a 40% female labour content. All these manufacturing centres contributed to the grand total of 168,040 Merlin engines produced, with Packard alone completing over 55,000 units in their immense plant in Detroit. Each factory played its own particular part in the production of this excellent war-winning engine. It perhaps should be stated at this point for absolute parity, that the Bristol Hercules engine was also a fine and reliable power plant, used in many British wartime and post war civil aircraft; a very worthy stablemate to the Merlin. Over 57,400 Hercules engines were built by Bristol and its shadow factories during the war years.
Not to be confused with the shadow factory system and quite separate from it was the concept of “dispersed” manufacture. Some pre-war planning initiated in 1937 with a government White Paper, recognised the need for the dispersal and duplication of manufacturing facilities, together with an appreciation of the possible consequences if a key factory suffered a “knockout blow” from enemy action. This scenario became a reality in September 1940 when the Vickers-Supermarine main headquarters and manufacturing plant at Woolston, Southampton, and its nearby new satellite the River Itchen works, were both severely damaged in enemy air raids. Both these key facilities were engaged in the manufacture of major Spitfire assemblies, including wings and fuselages. Final assembly was accomplished at nearby Eastleigh airfield, now Southampton Airport.
When Lord Beaverbrook, then Minister for Aircraft Production (MAP), visited Southampton after the devastating raids, he ordered the immediate dispersal of all Spitfire production to numerous sites across the counties of Berkshire, Hampshire and Wiltshire. The Supermarine Headquarters, including administration, design and experimental departments, were relocated to Hursley Park, a large country mansion to the south west of Winchester. As an adjunct to this edict, it was also ordered that other potentially vulnerable aircraft firms should disperse as many of their activities as possible, with the utmost urgency. The Gloster Aircraft Company, for example, eventually occupied over 40 dispersed sites in and around the Gloucestershire area. This even included using Nissan hutted buildings located in orchards!
Not all shadow factories were conventional structures constructed at ground level, several in fact were concealed underground. The concept of the underground factory stemmed from the early days of the war, when there was still a degree of uncertainty about the extent and destructive effects of enemy bombing on conventional installations. Underground factories fell into two basic categories;
- Those occupying former quarries, underground workings, etc, such as the ex-Bath Stone Spring Quarry, Corsham, Wiltshire, which was acquired by the MAP in 1940, and scheduled for shadow manufacture of the Bristol Hercules aero engine, BSA gun barrels and Dowty aircraft undercarriages.
- Purposely constructed underground facilities, such as the tunnels excavated in the sandstone hills at Drakelow, Wolverley, near Kidderminster, Worcestershire, which were designated for use by the Rover Car Company in its shadow manufacture of Bristol Pegasus and Mercury aero engines.

For various reasons underground shadow factories never really became the accepted norm in Britain. The considerable expense and protracted time in constructing and provisioning underground facilities rendered them less attractive to the government. By the time Corsham was ready for production, and the 3.5 miles and 54 acres of tunnels at Drakelow had been completed and machine tools installed in November 1942, the threat from massed air attacks on Britain had largely receded. Only a portion of the Drakelow complex was actually used by Rover for manufacturing purposes, the remainder being used by the RAF as a store. Corsham and Drakelow were thus very expensive government funded manufacturing facilities of somewhat questionable value. After the war some of the tunnels at Drakelow performed another function as nuclear bunkers in the Cold War era. Part of the Corsham complex still, it is believed, has an MoD presence.
The Austin Motors plant at Longbridge, Birmingham, also had a wartime underground facility for the assembly of Bristol Pegasus radial engines. By and large most shadow factories were situated above ground, and in many instances were camouflaged; the Avro Lancaster shadow factory at Yeadon, West Yorkshire, being an exceptionally good example of the camoufleur’s art.
In summation, the shadow factory system worked very well for Britain in the second world war. It fulfilled most of the demands made upon it and provided sufficient war materiel of the right quality when it was most desperately needed. During the Battle of Britain period, for example, there never was at any time a shortage of fighter aircraft, thanks in part to the shadow factory system. Of much greater concern to Sir Hugh Dowding, C-in-C Fighter Command, was the dwindling number of pilots. Similarly, the considerable output of four engined heavy bombers, notably the Stirling, Halifax and Lancaster, was facilitated by the extensive use of shadow factories. There are still a few remaining shadow factories performing manufacturing functions today, perhaps most notably the Castle Bromwich plant, now part of Jaguar. However, many more have disappeared in recent years, including the former Rootes Group Ryton on Dunsmore factory and the Coventry Banner Lane plant, previously occupied by Massey-Ferguson for tractor production. Both of these factories originated as “shadows” for aero engine manufacture. These two sites have now been redeveloped for modern housing.
A very good BBC documentary item called “Inside Story”, which featured archive footage of the Castle Bromwich aircraft factory, together with interviews with ex-personnel, including the late Alex Henshaw is available to watch at this link, click here.
References:
- Concealment and Deception, The Art of the Camoufleurs of Leamington Spa, 1939-1945, Exhibition Catalogue, 2016
- Drakelow Unearthed: The Secret History of an Underground Complex, Paul Stokes, 1996
- Hives and the Merlin, Sir Ian Lloyd and Peter Pugh, Icon Books, 2004
- Major Piston Aero Engines of World War II, Victor Bingham, Airlife Publishing, 1998
- Classic World War II Aircraft Cutaways, Bill Gunston, Bounty Books, 2011 Edition
- The Battle of Britain, New Perspectives, John Ray, Brockhampton Press,1994
- The Castle Bromwich Aircraft Factory, WIAS Paper, JF Willock, 2021
Copyright © J F Willock 2025