WWII fighter attack aircraft, cruising around looking for targets at about 500 ft and a minimum of 240 KIAS (which is 4 miles per minute). Much above 500 ft you can't see much in the way of tactical targets (aircraft in CMBB have really good eyesight). LOS horizon at 500 ft is about 27 nmi (with no obstuctions). So horizon to horizon is about 6 minutes. On a good day you can spot a fighter at about 10 nmi max (yes I know Chuck Yeager did 50 nmi once, that's why he's Chuck). Giving you about 2.5 mins of vulnerability. Inside of about 3 nmi the LOS angle changes are probably going to exceed any large gun's tracking capability (unless you are the target). So of the 2.5 minutes of vulnerability you have about 30 seconds to detect, track, and engage an inbound aircraft with any reasonable probability of kill and about the same for revenge shots as he departs.
All that being said, when there are aircraft overhead, everyone hides. When you are on the ground with aircraft attacking, I was taught that everyone shoots back, with every weapon you can bring to bear. Golden BB rule applies. More aircraft have been lost in every war to small arms fire than any other cause.
I haven't seen a vehicle mounted MG (or infantry)engage an attacking aircraft, will this happen in CMBB? Yes I know the Pk is fairly small, but I think it is historically accurate.
Feel free to correct me if this is full of dung, I hardly qualify as a grog.