GEBT: A general-purpose nonlinear analysis tool for composite beams. Geometrically Exact Beam Theory (GEBT), a general-purpose tool for nonlinear analysis of composite slender structures, is developed to meet the design challenges associated with future engineering systems featuring highly-flexible slender structures made of composites. GEBT is based on the mixed formulation of the geometric exact beam theory which can capture all geometric nonlinearities due to large deflections and rotations, subject to the strains being small. Coupled with Variational Beam Sectional Analysis (VABS), a general-purpose cross-sectional analysis, GEBT can effectively analyze geometric nonlinear behavior of slender structures having arbitrary cross-sections made of arbitrary materials.