This paper presents the outcome of a theoretical study of materially nonlinear flexural behavior and strength of a hybrid composite bridge girder that consists of a so-called Hillman composite beam and an integrated concrete bridge slab. The load versus mid-span deflection response of the bridge system was studied to determine whether the flexural behavior of the composite system aligns with conventional beam theory or if it is modified by the complex geometry of the beam, specifically the concealed arch. Theoretical load-deflection relations are developed by using the non-linear moment-curvature relations generated by the specifically formulated algorithms combined with the central finite-difference scheme and then compared with the available experimental results. To determine the relative contribution of the cross-sectional elements in the overall load-deflection response of the bridge system, load-deflection relations are developed for four different cross sectional configurations of the system. The study shows that the deflections given by beam theory at low loading are significantly higher than the experimental results confirming the existence of arch action in that loading range. An excellent agreement is, however, witnessed between the theoretical and experimental load-deflections relations at loads higher than the weight of the overlying bridge slab which proves the absence of arch action during the test loading of the specimen. The study also reveals that the concrete deck and steel strands jointly act to produce much of the internal resisting moment of the bridge girder whereas the contribution of arch, fin and the FRP box is less dominant.