Actually it was a no brainer, for the bomber.
Attack from the rear is so deadly to the fighter, which presents ZERO deflection and aim off, which is not the same thing, as to allow one B-17 tail gunner to shoot down an entire squadron of Me-109s! Their instructor and the 11 recruits who followed him!
Also, the front of the fighter with a liquid cooled engine is it's most vulnerable quadrant! One little tiny .30 cal bullet in the Radiator, Oil Cooler, Engine cooling jacket, gear case below the oil level line, oil pan below the level line, etc... Needless to say, a single .50 caliber would certainly hit more than one of those things! While some of those hits were not instantly disabling, they all forced a landing one way or the other. The Spit had two radiators under the wings with a cut out that prevented either one, but not both from downing the plane. The P-51 protected the radiator and cooler with the shape of the duct leading to them, but the engine was still a single bullet kill. The P-38 and 40 were even worse! They had extra coolers to hit! Only radial engined fighters could attack from the front WO too much concern.
The Nazis kept score of those sorts of things and determined that B-17s shot down more fighters than any other type of Allied plane! ( 12-13K IIRC!) B-24s were in second place, (7-8K, again IIRC!) with a huge gap until the distant third place fighter! Can you guess which one?