A high-fidelity numerical formulation is presented for the high-speed aeroelastic behavior of slender composite wings. The compressible flow is modeled using the 3-D Euler equations on a deform able mesh, and an asymptotic approximation of the 3-D kinematically-nonlinear equations of elasticity models the anisotropic slender structure. The transfer of the distributed loads and displacements at the fluid-structure interface is based on detailed 3-D representations of the deformed aerodynamic and structural domains. Finally, a time-domain solution is implemented for the closely-coupled fluid-structure interaction problem. This procedure handles the large deflections appearing in very slender wings under aerodynamic loads using a description of the deformation that includes all geometrically-nonlinear effects in the aeroelastic analysis. Using this approach, the static nonlinear aeroelastic response of a 16:1 half-aspect ratio wing is investigated for steady flight conditions. The impact of the detailed 3-D representation of the fluid-structure interface on the aeroelastic response is investigated. For that purpose, numerical results are compared to the representation of the structure using a geometrically- nonlinear 1-D beam model.