The airframe structural weight fractions set for the High-Speed Civil Transport (HSCT) are ambitious and will require significant improvements beyond the current state-of-the-art aluminum skin/stringer designs typically employed on commercial transports. The proper application of advanced materials and structures technologies will play a major role in determining the overall success of the program, both economically and technically. This paper discusses some of the conceptual design trades that were conducted for the structural concept downselections, along with the strategy used to reduce weights for the wing and fuselage primary structure. Sizing trends are plotted as normalized values relative to aluminum skin/stringer weights followed by a discussion of the results. The work was conducted by McDonnell Douglas Aerospace (MDA) under the NASA funded High-Speed Research Program.