The M3 was the first real anti-tank gun adopted by the US military, and it was not formally adopted until 1940 – and was thoroughly obsolete in Europe by 1942. The initial design was based on a pair of German PAK-36 guns, but in its production guise the M3 was a much simpler gun. It used a fully manual vertically-sliding breechblock (as opposed to the German semiauto breech). It fired a 1.9 pound armor piercing projectile at 2900 fps (860g @ 885 m/s), which was able to penetrate 1-2 inches of armor at 500 yards, depending on the type of shell used and the angle of the armor.
In Europe, this gun because obsolete very quickly, although it had a useful service life in the Pacific theater. It was used against light armored vehicles, pillboxes, and even infantry (with canister shot). This was also used as armament on the M3/M5 light tank (Stuart), the M3 medium tank (Lee/Grant), the M8 Greyhound armored car, and the M6 Gun Motor Carriage. It was a simple and reliable gun, just not powerful enough for antitank service. It was replaced by a 57mm gun copied from the British, which would serve until the end of the war.
I had the chance to do some shooting with an M3 recently, and it was very pleasant to use compared to other antitank guns. From behind the shield in particular, the muzzle blast was not bad at all, and recoil was minimal (note the slight rearward roll of the gun in the video, because the skids were not used). I had not expected to do any regular-speed filming at this particular shoot, and did not bring proper camera gear – I apologize for how much noise is in the background.