The design methodology of composites in aircraft structures normally follows a building block approach and validation of design allowables is carried out through coupon, feature, and full-scale component level tests. Another important aspect in the design of composites is to account for environmental degradation of strength and stiffness properties, especially under a hot-wet environment. This paper describes the methodology adopted for and the results obtained from full scale static load tests with and without pressurization, on two composite pressure test boxes, under room temperature and hot-wet environmental conditions. The test boxes representing a typical wing structure, were made of three bays with the middle bay consisting of composite spars and top and bottom skins. One of the two identical test boxes was subjected to almost saturation level moisture conditioning at 70°C and 85% RH in an environmental chamber. The test boxes were extensively strain-gaged with back to back strain gages fixed at pre-selected locations, and these were very carefully moisture-proofed to withstand the moisture-conditioning over long period and rigorous hot-wet test at 100 + 5°C and > 85% RH. A component test rig and an aluminum test chamber were specially designed to undertake these tests, which involved application of loads and reactions that cause combined bending and torsion of the test box. Finite element analysis using “ELFINI” had predicted the test box to be buckling-critical. Through actual testing it has been validated that the top skins of the test boxes do not undergo buckling up to design ultimate load (DUL) and that the ultimate failure loads were well above the DUL of the test boxes, both under room temperature and hot-wet environmental conditions.