These could be specially designed or modified from serial production aircraft.
New instruments wiring and equipment, a fuel system and piping, structural alterations to the wings, and other adjustments are needed for this adaptation.
The aircraft had a mid-fuselage cabin for test instrumentation and observers.
Twelve were built and provided to British aero-engine companies.
A large number of aircraft-testbeds have been produced and tested since 1941 in the USSR and Russia by the Gromov Flight Research Institute.