There were two Ju 88P models equipped for ground attack. The P-1 had a 7.5cm cannon mounted. The aircraft was badly overloaded and as already stated firing was a severe strain on the airframe.
The P-2 mounted two 3.7cm cannon in a ventral fairing. It was intended more as a bomber buster than a tank buster but it too was overloaded and too slow for its intended purpose.

What surprises me (or maybe not) is that the Germans didn't come up with a solution similar to the US one for mounting a large cannon on an aircraft. In the German case they could have mounted a modified 7.5cm leIG 18 on even a Ju 88 within the existing fuselage and used this with HE and HEAT to achieve a worthwhile tank buster. The gun would have been far lighter, had far lower firing stresses involved and would have been more flexible in the type of targets it could have engaged.

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