Gallium nitride nanowire light-emitting diodes are an emerging class of nanoscale light sources that provide new capabilities beyond those of planar LED devices. In this work, we present an approach for obtaining nanowire LEDs with both high-aspect ratio and core-shell structure. These LEDs are grown on silicon substrates by selective area epitaxy and with an N-polar growth orientation that facilitates highly selective core growth and fabrication of uniform shell layers. Discrete nanowire LEDs that emit in the near UV are demonstrated and are further shown to be potential candidates for visible-wavelength emission with high color purity and nanosecond modulation speeds.