A biaxial nematic phase with D2h symmetry has the mesogen's long and short transverse axes simultaneously aligned along the two orthogonal directors, n and m, respectively. Low-angle x-ray diffraction measurements in the nematic phase exhibited by three rigid bent-core mesogens clearly reveal its biaxiality. The diffraction results can be readily reproduced by ab initio calculations of molecular form factor assuming short-range positional correlations in the nematic phase with the apex of the core aligned along m.