As a weapon of war in World War II, the A-bomb was of greater shock than practical value. They were too complex to mass produce in the late 1940s, and by 1945, American and British bombers had pretty much devastated every German and Japanese city worth bombing.
The secret of American victory in World War II was quantity and quality. Copious amounts of weapons and equipment that not only overwhelmed and outmatched the Axis arsenal, but helped enable Lend-Lease allies like Britain and Russia to do the same.
Not that every U.S. weapon was great. The ubiquitous M-4 Sherman tank was plentiful but mediocre. Early U.S. fighters like the P-40 and P-39 were nothing to brag about (except in the hands of the Flying Tigers), while U.S. submarine torpedoes had a bad habit of not exploding until late 1943.