Yeah, it was the Ju-88 P series that tried 75mm, 37mm and 50mm cannon in a ventral pack... - It's as Lanc Erich say, they were very heavy and unweildy, explosive bolts were fitted in case of fighter attack or flak taking-out an engine...only good for 244 mph, but they were used in limited no.'s, with a degree of success by Panzerjager-Staffel 92, firing 2 rounds per pass;- first from a 1000 ft in a glide-dive, then at 250 ft, quite accurately, but they had a slow rate of fire and the inability to evade, rendered them just another 'passing phase' in the tank battles...