Petter initiated a design proposal with F W "Freddie" Page leading the design and Ray Creasey responsible for the aerodynamics. By July 1948 their proposal incorporated the stacked engine configuration and a high-mounted tailplane but was designed for Mach 1.5. As a consequence it had a conventional 40° swept wing This proposal was submitted in the November and in January 1949 the project was designated P.1 by English Electric. On 29 March 1949 MoS granted approval for English Electric to start the detailed design, develop wind tunnel models and build a full-size mockup. The design that had developed during 1948 evolved further during 1949. To achieve Mach 2 the wing sweep was increased to 60° with the ailerons moved to the wingtips. In late 1949 low-speed wind tunnel tests showed that a vortex was generated by the wing which caused a large downwash on the tailplane; this issue was solved by lowering its height below the wing. Following the resignation of Petter, Page took over as design team leader for the P.1. In 1949, the Ministry of Supply had issued Specification F23/49, which expanded upon the scope of ER103 to include fighter-level manoeuvring. On 1 April 1950, English Electric received a contract for two flying airframes, as well as one static airframe, designated "P.1."