We study electronic and geometrical properties of B-doped diamond in the framework of the density functional theory. The systems studied are simple-cubic (sc) BC7 and face-centered-cubic (fcc) BC15 and B2C14. In the case of BC7, we also study the effect of the tetragonal and rhombohedral distortions of the unit cell to be introduced to the B-doped diamond grown homoepitaxially on the diamond (100) and (111) films, respectively. In the case of fcc B2C14, on the other hand, the inhomogeneous doping effects are discussed from the viewpoint of the energetics. It is confirmed that in the homogeneous doping case, B doping gives rise to hole doping into the valence band, which should be responsible for the metallic and superconducting transport properties of the material. In the case of the inhomogeneous doping, two B atoms on the first-nearest neighbor sites as well as on the second-nearest neighbor sites are found to lower the Fermi-level density of states, which is unfavorable for superconductivity. Although tetragonal and rhombohedral distortions are found to give a sizable change to the electronic states near the Fermi level, the value of the Fermi-level density of states itself is found to remain almost unchanged. Finally, in order to study the fundamental-gap value of the heavily B-doped diamond, we study the electronic structure of the sc BC7 and fcc BC15 in the GW approximation. A large reduction of the gap value upon doping is confirmed.