Self assembled III-nitride nanowires are promising candidates for optoelectronic devices. The precise control of size and position of the nanowires is crucial for further applications. We demonstrate the selective growth of arranged GaN nanowires by plasma-assisted molecular beam epitaxy on an AlN buffer. The position of each nanowire is controlled by a thin silicon oxide mask, patterned by electron beam lithography. The dependence of selectivity and nanowire morphology on the growth parameters and mask properties are investigated. We change the substrate temperature and the Ga-flux, retaining nitrogen rich conditions, which are suitable for self-assembled nanowire growth. Samples with different masks are produced, varying the thickness and the layout. The diameter of the holes and their distance from each other vary across the pattern. We discuss the influence of these parameters on the nanowire growth and morphology.