High Quality Content by WIKIPEDIA articles! This is a partial list of the British Air Ministry (AM) specifications for aircraft. A specification stemmed from an Operational Requirement, abbreviated "OR", describing what the aircraft would be used for. This in turn led to the specification itself, e.g. a two-engined fighter with 4 machine guns. So for example, OR.40 for a heavy bomber led to Specification B.12/36. Aircraft manufacturers would be invited to present design proposals to the Ministry, following which prototypes of one or more of the proposals might be ordered for evaluation. On very rare occasions, a manufacturer would design and build an aircraft using their own money as a "Private Venture" (PV). This would then be offered to the Ministry for evaluation. If the aircraft generated interest in the Ministry or RAF due to performance or some other combination of features then the Ministry might well issue a specification based on the Private Venture aircraft.