You want a slot drill, not an end mill. Slot drills look like endmills
but have one or two teeth that extend to the centre. They're used for
cutting keyways in shafts, or slots in flat stock; they'll drill
through vertically then can be run sideways like an endmill.
But it will take a really good, heavy drill press to keep it from
walking around. Better to use it to clean out and bottom a
previously-drilled, slightly undersize hole.
Dan