Shells of zero Gaussian curvature, ellipsoidal and combined pressure vessels made of fibrous composite materials being under the influence of constant internal pressure are considered. Within various shell theories, the influence of choice of composite material structural models, reinforcement parameters, and mechanical characteristics of a composite material on calculated values of stress-strain state for pressure vessels is investigated. It is shown that the use of simple theories can lead to a significant overestimation of structure bearing capacity. The influence of choice of structural models on the stress-strain state is much stronger for materials with the same-order Young’s moduli of reinforcement and binder. Proper choice of structural parameters can provide the best operating condition of composite materials.