One–dimensional model is presented for the analysis of thin–walled composite beams. Each wall is made of orthotropic layers bonded together to form a laminate that can be anisotropic. The theory uses the Navier–Bernoulli and Vlasov models to describe bending and twist, respectively, at beam level, and the Love–Kirchhof model to define the constitutive equations at lamina level. As first result, a 5 × 5 cross– sectional stiffness matrix is obtained that relates one–dimensional generalized beam forces and moments to one–dimensional generalized displacements. Later, the cross–sectional stiffness matrix of one beam element is obtained using The Virtual Work Principle and the appropriate shape functions. This formulation allows the modeling of either open–section or closed–section beams of arbitrary section shape with arbitrary layup. Two examples of sections with circumferentially uniform stiffness (CUS) and circumferentially asymmetric stiffness (CAS) are presented for the study of extension–twist and bending–twist elastic couplings, respectively. The technique has been validated comparing the results obtained with the results deduced by other authors.