1. A computer-implemented method of designing a virtual model of a aircraft blended body structure having two or more layered components for use in manufacture of the structure, wherein the method is implemented in a computer system comprising one or more processors configured to execute one or more computer program modules, the method comprising the steps of: defining a parameterised, theoretical procedural skin representing an external shape of the aircraft blended body structure; generating at least two datum skins from the procedural skin, each datum skin defining a working surface of a component in the aircraft blended body structure; defining at least one primary ply zone surface; defining at least one primary drop slope surface wherein the surface of the drop slope intersects the surface of a ply zone along a joint line; and defining a secondary ply zone surface as a normal offset to each primary ply zone surface and a secondary drop slope surface as a normal offset to each primary drop slope surface.