British shadow factories were the outcome of the Shadow Scheme, a plan devised in 1935 and developed by the British Government in the buildup to World War II to try to meet the urgent need for more aircraft using technology transfer from the motor industry to implement additional manufacturing capacity.
The term 'shadow' was not intended to mean secrecy, but rather the protected environment they would receive by being staffed by all levels of skilled motor industry people alongside (in the shadow of) their own similar motor industry operations.
A directorate of Aeronautical Production was formed in March 1936 with responsibility for the manufacture of airframes as well as engines, associated equipment and armaments. The project was headed by Herbert Austin and developed by the Air Ministry under the internal project name of the Shadow Scheme. Sir Kingsley Wood took responsibility for the scheme in May 1938, on his appointment as Secretary of State for Air in place of Lord Swinton.
Many more factories were built as part of the dispersal scheme designed to reduce the risk of a total collapse of production if what would otherwise be a major facility were bombed. These were not shadow factories, though some now use that name believing shadow refers to attempts to achieve a level of secrecy.
It was impossible for these facilities to be secret, though they were camouflaged after hostilities began. They were war material production facilities built in "the shadow" of motor industry plants to facilitate technology transfer to aircraft construction and run, for a substantial management fee, in parallel under direct control of the motor industry business along with distributed facilities. General Erhard Milch, chief administrator of the Luftwaffe, was in Britain again in the autumn of 1937 inspecting new shadow factories in Birmingham and Coventry, RAF aeroplanes and airfields.
Up until the middle of 1938, the Air Ministry had been headed by Lord Swinton. He had been forced by Prime Minister Neville Chamberlain to resign his position due to a lack of progress in re-arming the Royal Air Force, the result of obstruction by aging Lord Nuffield. Swinton's civil servants approached their new boss, Sir Kingsley Wood, and showed him a series of informal questions that they had asked since 1935 on the subject, such as those posed to Morris Motors with regard to aircraft engine production capability at their Cowley plant in Oxford. As it turned out, the specialised high-output engines required by the RAF were made by Armstrong Siddeley, Bristol Aeroplane, Napier and Rolls-Royce, all of which employed a high number of sub-contractors. Despite their new factories, protestations by Wolseley Aero Engines (Nuffield) and Alvis were ignored. Their products were not required. Engines were specified by the aircraft's designers. Nuffield did participate after Wood's appointment, providing the Castle Bromwich factory, but after two years management was so poor that not one Spitfire had been produced there. Castle Bromwich was withdrawn from Nuffield and placed under the wing of Vickers.