A design optimization study of a composite wing box for the civil tiltrotor transport aircraft based on a reduced wing maximum-thickness-to-chord ratio is considered. The wing box is designed for minimum structural weight subject to local constraints, as well as global constraints based on wing primary natural frequencies and overall buckling strength. The six loading conditions correspond to airplane, helicopter, and conversion flight modes. The continuous design variables include the ply orientation angle and thickness in each skin panel, the web thicknesses of the spars and ribs, and the cross-sectional areas of spar caps and stringers. The number of stringers is a discrete design variable. The structural analysis is based on FEM, and the optimization solution is based on the method of modified feasible directions. The results reveal that it is possible to minimize the structural weight, and arrive at a set of feasible wing designs, based on the number of stringers used, at a reduced maximum thickness/ratio; the loads corresponding to the airplane mode along with the wing torsion natural frequency have the greatest influence on the design. (Author)