The German military was a pioneer in the use of airborne operations, conducting several successful operations during the Battle of France in 1940, including the use of glider-borne troops in the Battle of Fort Eben-Emael. Impressed by the success of German airborne operations, the Allied governments decided to form their own airborne formations. This decision would eventually lead to the creation of two British airborne divisions, as well as a number of smaller units. The British airborne establishment began development on 22 June 1940, when the Prime Minister, Winston Churchill, directed the War Office in a memorandum to investigate the possibility of creating a corps of 5,000 parachute troops. In 1941, the United States embarked on a similar programme.
When the equipment for the airborne forces was under development, it was decided by War Office officials that gliders would be an integral component of such a force; these would be used to transport troops and heavy equipment. The first glider to be designed and produced was the General Aircraft Hotspur, the first prototype of which flew on 5 November 1940. However, several problems were found with the Hotspur's design, the primary one being that the glider did not carry sufficient troops. Tactically it was believed that airborne troops should be landed in groups far larger than the eight the Hotspur could transport, and also the number of aircraft required to tow the gliders needed to carry larger groups would be impractical. There were also concerns that the gliders would have to be towed in tandem if used operationally, which would be extremely difficult during nighttime and through cloud formations. So it was decided to use the Hotspur as a training glider, and continue with the development of several other types of glider, including a 25-seater assault glider which became the Airspeed Horsa.
The Horsa, given the designation of AS 51, was produced to meet Specification X.26/40 issued on 12 October 1940. Initially it was planned that the Horsa would be used to transport paratroopers who would jump from doors installed on either side of the fuselage, and that the actual landing would be a secondary role; however the idea was soon dropped, and it was decided to simply have the glider land airborne troops. An initial order was placed for 400 of the gliders in February 1941, and it was estimated that Airspeed should be able to complete the order by July 1942. Enquiries were made into the possibility of a further 400 being produced in India for use by Indian airborne forces, but this was abandoned when it was discovered the required wood would have to be imported into India at a prohibitive cost. Five prototypes were ordered with Fairey Aircraft producing the first two prototypes for flight testing while Airspeed completed the remaining prototypes to be used in equipment and loading tests. The first prototype (DG597) towed by an Armstrong Whitworth Whitley took flight on 12 September 1941 with George Errington at the controls, 11 months after the specification had been issued.
As specified in Specification X3/41, 200 AS 52 Horsas were also to be constructed to carry bombs. A central fuselage bomb bay holding four 2,000 lb or two 4,000 lb bombs was fitted into the standard Horsa fuselage. The concept of towing bombs was dropped as other bombers became available, with the order for the AS 52 cancelled.
Production of the Horsa commenced in early 1942, and by May some 2,345 had been ordered by the Army for use in future airborne operations. The glider was designed from the outset to be built in components with a series of 30 sub-assemblies required to complete the manufacturing process. Manufacturing was intended primarily to use woodcrafting facilities not needed for more urgent aviation production, and as a result production was spread across separate factories, which consequently limited the likely loss in case of German attack. The designer A. H. Tiltman said that the Horsa "went from the drawing board to the air in ten months, which was not too bad considering the drawings had to be made suitable for the furniture trade who were responsible for all production."
The initial 695 gliders were manufactured at Airspeed's factory in Christchurch, Hampshire, with subcontractors producing the remainder. These included Austin Motors and the furniture manufacturers Harris Lebus. The subcontractors did not have airfields to deliver the gliders from, and sent the sub-assemblies to RAF Maintenance Units for final assembly. Between 3,799 and approximately 5,000 Horsas were built when production ended.