We have investigated the growth of GaAs nanowires as a function of temperatures and source pressures on (111) B-oriented substrates in chemical beam epitaxy (CBE), to establish the mechanisms that govern wire growth and to optimize growth conditions. The grown nanowires were characterized with a scanning electron microscope (SEM). We found two mechanisms to be of importance for wire growth: (i) sufficiently long diffusion length of the group-III material on the 2D substrate surface and on the side facets of the nanowire to obtain rod-shaped nanowires and (ii) growth conditions that suppress growth rate on adjacent surfaces to enhance the wire growth. Favorable conditions for these mechanisms are growth temperatures between 515 and 535°C, and As-rich growth conditions. Furthermore, we suggest that the growth mechanism of nanowires in CBE is based on surface-selective-growth (SSG) with a solid seed particle rather than conventional vapor–liquid–solid (VLS) growth.