The first attack helicopter

It is entirely plausible that the world's first attack helicopter could have been invented during WW 2. I would say this would be the invention of some enterprising officer and technicians in the field. Much like mounting bazookas or panzerfaust on light aircraft, what happens is this:

Some field unit has been issued the Sikorsky R-4 helicopter, the only production helicopter of WW 2.



This is a two-man helicopter that had about a 250 lbs. load capacity. What the unit in the field does is add a .30 Browing M1919 or M2/AN machinegun fixed to fire forward between the crew seats with an ammunition feed for box of about 300 rounds of ammunition at a total installed weight of about 80 lbs. The gun is either electrically or manually fired by the pilot.
In addition, a rig of 2 x 4 bazookas are added, 4 per side on a right to the landing gear struts, placing them away from the fuselage and these are angled in very slightly to converge at say 200 to 300 yards out. These are also fired by the pilot electrically and can be salvoed in pairs. Total weight of the installation with 8 rounds of ammunition is about 140 lbs.
Some sort of gunsight is installed in front of the pilot's position that allows for reasonably accurate aiming of the weapons. All you'd need are a vertical line with horizontal range lines painted on the windshield, one set for the bazookas, another for the .30 cal.

That's just over 200 lbs. of ordinance and mountings added to the helicopter with one crewman aboard.

Against tanks or vehicles not defended by dedicated AA guns, the crew uses the .30 to suppress return ground fire and close with the target, then uses the bazookas to destroy it. Unlike an airplane, the pilot can set their helicopter down on open ground, dismount to scout, and then get airborne to attack targets. They can use ground cover and their better maneuverability to stalk targets and attack from surprise locations.

Thus, the R-4 could have become the world's first attack helicopter. As it is already in production, once this gets back to the higher powers, they might up production and get a proper design version in production as well.