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During the pre-/early-war period the Do-17’s defences were quite adequate, especially when combined with the aircrafts exceptionally high speed. You have to keep in mind that during the 30s there was a sentiment that bombers would be safe from fighters du to their higher speed alone. Seems silly in hindsight but it was widely accepted and the Do-17 was specifically build as a “Schnellbomber” with this concept in mind. It sounds less outlandish when you consider that most air forces were still using biplanes at the time. The RAF was still primarily equipped with Furys and Gladiators when the Do-17 entered service. Against those kinds of opponents it *was* a credible defence.
The Spitfires and Hurricanes the Do-17 had to fight during the Battle of Britain where a whole new generation of fighters against which it had considerably lower chances of survival.
Fast forward to 1944, B-17s vs. Bf-109s and Fw-190s over the Reich. Here we have fighters and bombers from roughly the same timeframes and the outcome is not nearly as clear-cut. Well maintained formations of Flying Fortresses had good chances of surviving fighter attacks with…acceptable losses, even without escorts. Enter the Me-262: another new generation, same situation. Bomber defences are nearly useless, the speed advantage was just to great. Interestingly enough the old 1930s concept of the fast bomber becomes viable again at this point: the Ar-234
as the world’s first jet powered bomber was once again fast enough to effectively avoid interception.
**TL;DR:** Bomber defences are not generally ineffective but they are drastically reduced in their usefulness when pitted against significantly newer opponents.