The paper describes a procedure, dedicated to the preliminary sizing of an aircraft wing-box, based on a two level decomposition and on two independent optimizations. An engineering approach is followed and the most suitable model is exploited at each level in order to generate information about the wing-box structure without the need of a previous knowledge, a very important feature in the case of unconventional structures. The first level uses a commercial structural multidisciplinary optimization code and a stick model of the wing primary structure to minimize the structural mass under global design constraints; a set of physical design variables are used referring to a schematic representation of the cross-section in terms of equivalent axial thickness supporting pure axial stress and the thickness of a box resisting to both axial and shear stresses. The second level, based on a genetic optimization, provides the minimum mass optimal design of the wing cross sections, in terms of local design variables, which safely support the internal loads supplied by the first level, under local constraints, e.g. panel buckling and stiffeners crippling, providing also a cross section stiffness in compliance with the first level. The reported example, concerning the B747-100 wing structure, shows the capability of the approach to predict the structural weight of the wing box, an information to be used mainly in an early stage of the aircraft design, and to suggest a set of cross sections design solutions all in compliance with both global and local requirements
