Belly windows in F4F-3s, -3As, -4s, FM-1s, and some of the early -2s dated back to pre-war thinking for a limited dive bombing capability in fighter aircraft. F4F type Wildcats could, and did, carry small bombs under the wings, as I recall, somewhere in the neighborhood of 100# each, one under each wings. According to an F4F driver of my acquaintance (who was in VS-41 flying SBUs in 1940 when it was re-designated VF-42 and everyone transitioned to the F4F-3, ergo, he knew more than just a little about dive-bombing practices) the best way to execute such an attack in an F4F was to fly over one's target and when you could first see it appear through the belly windows that was the time to roll over and execute your attack. The caveat was that such attacks were pretty much by "seaman's eye" since the F4F had no sighting capability for bombing.
In practice, there was little use for the capability. At Lae-Salamaua in March 1942, for example, the VF-3 F4Fs off Lexington were, indeed, equipped with bombs and executed attacks on various anchored Japanese shipping with little to show for the effort. At the same time, the also present Yorktown VF-42 F4Fs not only did not carry any bombs, but the Yorktown air staff had long ago decided that this was a pretty pointless exercise for F4Fs and had removed the hard points for carrying pip-squeak bombs, leaving the planes for their primary purpose, fighters.
Of course, the irony was that by the end of the war USN F6F and F4U fighters were routinely hauling 1,000 lbs of bombs as a matter of course and FMs up to 500 lbs. These, however, were, for the most, part delivered to customers in a glide-bombing attack profile as opposed to true dive-bombing and the airplanes were equipped with sights that could be set for bombs, rockets, or guns.