The system of producing aircraft to a specification ran from 1920 to 1949 during which the Air Ministry was replaced by first the Ministry of Aircraft Production (MAP) and then the Ministry of Supply (MoS). The system was applied to commercial aircraft as well, two being the de Havilland Comet and Vickers Viscount. During the period, over 800 specifications were issued.
Each specification name usually followed a pattern. A leading letter was usually present to identify the aircraft purpose. The codes used included B for "heavy bomber", e.g., B.12/36, P for "medium bomber", e.g., P.13/36, F for "fighter", e.g., F.10/35, and A for "army co-operation", e.g., A.39/34. The second part was a number identifying it in sequence and then after the slash, the year it was formulated, so in the example given above, B.12/36 signifies a specification for a heavy bomber, the twelfth specification of all types issued in 1936. Specifications were not always issued in sequence.
Admiralty specifications were identified by the letter N (Naval), e.g., N.21/45, and experimental specifications identified by the letter E (Experimental), e.g., E.28/39, with training aircraft signified by the letter T (Training), e.g., T.23/31, and unpowered aircraft, signified by the letter X, e.g., X.26/40. The letter G (General) signified a general-purpose aircraft, e.g., G.9/45, with an M (Multi-role) being applied to aircraft intended for more than one specific purpose, e.g., M.15/35.
The letter C (Cargo) was applied to military transport aircraft, e.g., C.1/42, with the letter O (Observation) used for a naval reconnaissance aircraft, e.g., O.8/38 - the letter S (Spotter) used for the more specialised role of naval spotting, i.e., observing and reporting back the fall of naval gunfire, e.g., S.38/34 - and R (Reconnaissance) for a reconnaissance type - often a flying boat, e.g., R.3/33. Special purpose aircraft would be signified by a letter Q, this being used to specify aircraft such as target-tugs, radio-controlled target drones, etc., e.g., Q.32/55.
Sometimes the purpose for which an aircraft is used in service would change from that for which the specification to which it was designed was issued, and so there are some discrepancies and inconsistencies in designation, the Royal Navy in particular liking to specify multiple roles for its aircraft in an attempt to make the best use of the necessarily limited hangar space onboard its aircraft carriers. In this case this resulted in several types designed to specifications originally intended to signify the naval Spotting role also being used for other purposes, e.g., S.15/33, resulting in the Blackburn Shark and Fairey Swordfish, the latter aircraft being primarily utilised as a torpedo bomber. Similarly S.24/37, which produced the Fairey Barracuda, again primarily designed for spotting, the dive bomber/torpedo bomber requirements being regarded as secondary when the specification was issued, but for which roles it was almost exclusively subsequently used, the original spotting requirement having been made obsolete with the introduction of radar.