High-strength steels (HSS) are increasingly used in structural engineering, particularly in long-span bridges and tall buildings. Simultaneously, hybrid structures combining HSS and normal-strength steel (NSS) are gaining popularity due to their efficient material usage. In hybrid girders, high-strength steel is placed in the flanges to resist bending moments, while the webs are made of NSS, which adequately resists shear forces. This configuration allows for reduced cross-sectional dimensions, more slender structures. These benefits translate into material savings, lower transport and construction costs, and reduced CO₂ emissions. However, the theoretical and practical framework for such hybrid girders is still limited in the international literature. Therefore, a combined experimental and numerical research program is carried out, focusing on the bending and bending-shear resistance of hybrid box-section girders. Six full-scale laboratory tests are conducted, and a total of 20 numerical models are evaluated (12 under pure bending and 8 under combined bending and shear), also incorporating data from previous experimental studies. The flanges use S700, S500, S460 or S355 steel, while webs are made of S355 or S235; reference specimens are homogeneous girders made entirely of S355 or S700. This study presents the detailed test setup, failure modes, resistance values, and results of numerical tests.