The paper describes current trends in the design of load-bearing structural elements of modern airliners made using polymer composite materials. In modern passenger airliners, polymer composite materials are used to ensure mass perfection of both lightly loaded elements and critical units, including wing and fin boxes. By means of the finite element method, a box model was created using shell finite elements. Parametric optimization was carried out with account for the anisotropic properties of structural materials. The purpose of the work is to develop a methodology for calculating the structural strength elements of the box and justify the required thicknesses with restrictions on compressive / tensile deformations and loss of stability. The developed methodology makes it possible to obtain a box structure with a minimum mass, while maintaining the necessary stiffness and strength characteristics at the stage of preliminary design.