I don't recall the specifics of NW 255, but stall can result from slowing to that speed or stall speed can be increased to above the current airspeed by increasing the loading on the wing.

The first is the standard stall - keep reducing speed without changing the flight path (it doesn't matter if the flight path in a climb, level flight, or descent). Decreasing speed reduces lift, which is compensated for by increasing angle of attack. The wing will stall at the speed where the angle of attack equals the AoA where airflow separation occurs. But that's the standard stall speed.

The second is the accelerated stall - the stall speed increases because the wing loading is increased.

Jim