This paper presents a flexural–torsional analysis of thin-walled composite box beams. A general analytical model applicable to thin-walled composite box beams subjected to vertical and torsional loads is developed. This model is based on the shear-deformable beam theory, and accounts for the flexural–torsional response of the thin-walled composites for an arbitrary laminate stacking sequence configuration, i.e. unsymmetric as well as symmetric. The governing equations are derived from the principle of the stationary value of total potential energy. Numerical results are obtained for thin-walled composites under vertical loading, addressing the effects of fiber angle and span-to-height ratio of the composite beam.