June, 1938 was when the first grown up Superman appears. He would have come to earth in the 19-teens.
Also, it is my understanding that, originally, Superman wasn't nearly as powerful as he is now. At first he could over jump over buildings - now he can fly. And before he could withstand blasts up to a Howitzer, but now he can take a nuclear blast and laugh.
That is correct. The explanation of his powers changed, too. At first he was just an evolutionarily advanced Kryptonian transplanted to Earth. Then Earth's weaker gravity was used to explain his super-strength. Then they threw in Earth's yellow sun super charging him(which didn't explain his ability to fly through solar systems at near light speeds).
He eventually developed a "code against killing", which would crimp his style in war time. But in the war ... check out this cover:
