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An asymptotically correct model for initially-twisted, thin-walled, composite beams has been constructed by the variational asymptotic method. The strain energy of the original three-dimensional structure is first rigorously reduced to be a two-dimensional energy expressed in terms of shell strains. Then the two-dimensional strain energy is further reduced to be expressed in terms of the classical beam strain measures. The resulting theory is a classical beam model approximating the three-dimensional energy up to the first order of the initial twist. Consistent use of small parameters that are intrinsic to the problem allows a natural derivation for all thin-walled beams within a common framework, regardless of whether the section is open, closed, or strip-like. Simple examples are provided to demonstrate the usage of the developed theory and compared with a numerical approach.