The Kestrel was a step between the straight-winged Hawker P.1127 prototypes and the operational Harrier. This aircraft wears the awkward markings of the Tripartite Evaluation Squadron, which consisted of British, US and West German personnel. In 1965 the projected Mach 2 Harrier was cancelled, and the Harrier ended up as just a straightforward development from the Kestrel.