The M134 GAU-17 “Vulcan ... the 20mm version was selected for further testing to determine its suitability as an aircraft mounted weapon. In 1956 the gun was standardized as the M61 20mm ...

Instead, the Air Force provided the smaller 20mm M61 Vulcan cannon that was in widespread use for the head-to-head trials, which the Fairchild Republic design ultimately won. So, Northrop’s Y-A ...