There were plenty of other reasons to attack from below - interest in the tactic predates the war - but I don't think the tactic would have been anywhere near a

There were plenty of other reasons to attack from below - interest in the tactic predates the war - but I don't think the tactic would have been anywhere near as effective if RAF heavy bombers had been equipped with belly turrets (and armed with something better than the basically useless .303 machine guns). Entirely different tactics were used against the USAAF formations in daylight, where a slow overtaking approach from below would be far to vulnerable. whereas detecting a plane by looking downwards towards the dark earth was almost impossible This would be true on the long approach to the target, but once near the target area, bombers would be silhouetted from below by searchlights, flares, and the burning city beneath them. Enough light was available from these sources for the Luftwaffe to use Wilde Sau single-seat "day" fighters at night.