lso you have to question the effctiveness of defensive weapons. When used in daylight they proved to be realtively useless TBH - for a start machine guns v cannons was no match
The main benifit of the air gunner was visual warning - which, at night, was highly variable - and the morale effect of firing back
Of course a downward and backward radar set or radar warning receiver was needed but the active sets were large & cumbersome and would have given the enemy somethnig to home on once they figured out they were fittted. There were warning receivers but they were always slightly behind enemy advances