A method of designing a virtual model for the manufacture of a blended body structure having two or more constituent elements, the method comprising the step of defining a parameterised, theoretical procedural skin from which two or more datum skins are generated, each datum skin defining a production outline of an airframe structure constituent element. The use of a procedural model with embedded parameterisation and associativity to derive the datum skins reduces the time taken to produce the datum skins and enables feature modifications to be incorporated into models quickly and consistently.