To design and aircraft and show that it will fly, you want X-Plane. In FS2004 you could take a 3D model of a brick and attach the Concordes perfromance data to it.
Burt Rutan and others have used X-Plane for training and/or modelling to show that a new design will fly. If you want you could even design an aircraft to fly on Mars as X-PLane include Mars' terrain data and atmosphere characteristics. I wouldn't expect it to be good enough to compete with whatever programs Airbus, Boeing and NASA use, but for preliminary tests it seems to be good enough for smaller companies.
X-Plane
goates