In this paper we report a theoretical study of the effects of the presence of boron in growing carbon nanotubes. We employ a well established tight binding model to describe the interactions responsible for the energetics of these systems, combined with the molecular dynamics simulation technique and structural relaxation calculations. We find, in agreement with the previous theoretical/experimental work of Blase [Phys. Rev. Lett. 83, 5078 (1999)], that boron favors (n,0) (zig-zag) tubular structures over (n,n) (arm-chair) ones by stabilizing the zig-zag edge. Furthermore, it is shown that boron has the effect of delaying the tube closure process, a fact which could explain the improved aspect ratio experimentally observed in nanotubes synthesized in the presence of boron. Our dynamical simulations lead us to propose a mechanism through which this extension of the closure time can be explained.