The RAF held the view (until the P-51 proved them wrong) that escorting bombers with fighters would never work, because to be able to carry enough fuel to meet the range requirement, the escorts would have to be bigger and slower than the enemy interceptors. They also did not anticipate the Luftwaffe being able to launch their escort fighters from bases in France.

Upward-firing guns were first used in WW1, with experiments continuing in the interwar years. There were plenty of examples for the Luftwaffe to study.

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Tony Williams: Military gun and ammunition website