I'm quite sure most canopies were bulletproof plexiglass, but as Dutchmn said, "bulletproof" does not really mean bulletproof, more like bullet resistent. Impacts at shallow angles will tend to bounce off, but at steeper angles a bullet will simply embed in the glass, which is made not to shatter. Given the thickness of canopy glass, at very steep angled impacts, near 90 degrees, a bullet or shell would most likely pass through. It takes a very thick pane of bulletproof glass to completely prevent penetration at all angles.
The bulletproof glass used in CIA armored SUV's, for example, is several inches thick, and it is simply designed to be immune to all assault rifle fire. No guarantees for heavy machinegun (.50) fire.