First-principles total-energy calculations for solid boron predict a transition from the insulating \ensuremath{\alpha}-rhombohedral phase first to a metallic body-centered-tetragonal structure at 210 GPa and subsequently to a face-centered-cubic structure at 360 GPa. While the possibility of intervening phases not considered here cannot be ruled out, it is argued that 210 GPa is an upper bound for the onset of a sequence of structural transitions by which boron evolves from a covalent insulator to a trivalent metal more like its neighbor Al.