Pixel and voxel-basis are commonly used for designing X-ray CT projectors that underly existing projection and back-projection methods that are widely used in practice. We introduce a bilinear projector for modeling X-ray optics and provide experimental evidence showing its impact in reducing discretization artifacts in Model-Based Iterative Reconstruction (MBIR). We demonstrate the advantages of using this bilinear model over the conventional pixel-based projectors (piecewise constant) for MBIR in realistic simulations in presence of nonlinearity of the Lambert-Beer law. We derive efficient piecewise polynomial forms for footprint and detector blur computations for this new projector and demonstrate its efficient GPU implementation for high resolution fanbeam geometry.