The common belief that the US found the .50 adequate and didn't bother with cannons is completely wrong. Even pre-war the USAAF wanted cannon armed planes; the P-38 and P-39 were both designed with cannon armament. However, the Americans had real problems making their cannons work properly. The Oldsmobile cannon in the P-39 often could only get a shot or two off before jamming, and the US 20 mm Hispano development was a complete debacle of incompetence, institutional bickering and Anglophobia. So when war came the USAAF had to resign any hopes of cannon armed fighters and focus on the only reliable gun they had: The Browning M2. It was overweight (weighed almost as much as a 20 mm MG 151), but it was reliable and packed a good punch for the size of its round.
