Originally posted by Dr Zhivago
And why Ju88 got only one mg firing forward 
:(
Real armament of Ju88 A4 was two MG 81 or one MG 81 and one MG 131 firing forward, twin MG 81 or one MG 131 upper rear, one or two MG 81 at rear of ventral gondola and (later aircraft) two MG 81 at front of gondola
I watched Discovery Wings Channel last night, the featured aircraft on Wings of the Luftwaffe was the He-111.
A few reasons for light defensive armaments in Luftwaffe medium bombers stem from the fact that the He-111 and Do-17 were designed in the early 1930s as commercial passenger/cargo aircraft to meet the treaty limitations.
The Luftwaffe High Command believed that a fast bomber like the He-111 or the later JU-88 should be able to out run fighters.
At the start of the war the He-111 could do exactly that.
Also the Luftwaffe worked so closely with the army that medium bombers nicely filled the support role required. The He-111 could even dive bomb, although level bombing worked better.
And as you add defensive arms to an aircraft , you increase weight and drag as well as decrease range payload, and manueverability.
The show went on to speculate that had the Germans developed and produced heavy four engine bombers like the Ural Bomber, the Germans could have chased the factory equipment the Russians moved east of the Urals.
With as few as a 1000 heavy bombers they may have been able to hamper the Russian weapon production just enough as to extend the war another twelve months.
Allowing the German Jet and Rocket programs time to stem Allied air power and slow the ground advance, which may have resulted in the Germans having the ability to surrender much as they did in WW1, with thier country intact.
Interesting hypothosis, however I believe the Allies wanted the same unconditional surrender from the Germans that they wanted from the Japanese, and so although heavy bombers may have extended the war, raised the death toll, and allowed the Me262 to slow the Allied Air Forces, utimately the Allies still would have pushed thru to Berlin to insure Hitler's removal.
As for the On-Topic response The pilots of WW1 were grateful when A/C started having MGs mounted facing forward, as this allowed them to point the plane at the target.
Imagine you're in a turret behind the pilot, he's jinking and diving, climbing and rolling, he wants to keep from getting shot down.
You're in the back trying to line up a shot, and he's twisting the plane around, throwing off your aim 99% of the time.
Being a gunner in WW2 would not have been a whole lot different, although a pilot would not move the A/C so radically, the pilot is gonna still have A/C buffet, side slipping and avoidance of falling debris to contend with.
Does it sound easy?
Makes ya kinda wonder how many WW1 gunners reached around and slapped the pilot in da back o da head don't it?