Ju 87 D has relatively modest improvement in attack stats from B version except naval defense which doubles from 6 to 12. This is odd and would seem to be the reason Ju 87 B is so vulnerable to naval return fire in early war scenarios. Ju 87 B should have a higher naval defense rating.
Also, naming convention is different from B to D which looks odd in game as well.
I would prefer naval attack to increase rather than naval defense.
The German air types are almost useless in naval attacks, only the Ju87 has some sting.
B version can inflict damage on naval targets but with high losses in return, even for elite units. But my main point is nothing really changes from B to D variant that would make it more or less vulnerable to return fire. All other German types of the time (Bf 109, Bf 110, Do 17, He 111) have a 10 for naval defense. For some reason Ju 88 only has 7 rating which you would think would be higher since it is the fastest bomber and not really used as a dive bomber at the time. Ju 87 B should have at least a 10 rating like others and Ju 88 should be higher as well.
Normally, the Ju 87D variant was a significant improvement over the B. While the B could carry a 500 kg bomb or up to 700 kg in smaller bombs, the D could carry a single 1000 kg bomb or a mix of smaller bombs to carry up to 1500 kg. That was an impressive doubling of its armament.
The naval long-range variant R with drop tanks between B and D was restricted to carry only a single 250 kg to improve its range to almost triple the range of the B.
Unlike B and R, the D variant received self-sealing tanks which definitely improved its survivability, although its overall lack of speed was still a hindrance to switch over to the faster ground-attacking Fw 190F/G in ‘43, even though with similar load capability of the Ju 87B variant again.
The stat progression of infantry, tank and aircraft variants in OOB usually looks like that it doesn’t necessarily reflect much reality, but is rather meant to give a reason to upgrade. The Ju 87D’s naval defense jump from B variant with 6 to 12 is indeed quite fantastic.
I guess someone noticed that the increasing AA attack-capability of naval ships over the war couldn’t compete anymore with a defensive rating of 6-7 and boosted the defense accordingly.
Technically seen, the Stuka should still be more fragile compared to the Ju 88 according to mass and little armor they carried, although the dive attack of the Stuka is more accurate and difficult to be intercepted by AA compared to the possible/stable glide-bombing of the Ju 88.