The Atom's original costume featured  a full face mask, leather wrist bands and shorts and bright red boots.  He looked like a professional wrestler. 
 

From All- American 24

In Flash Comics 98, August 1948, he received a new costume, courtesy of Gil Kane.  At approximately the same time, he also developed super-strength through means that were never explained.

This image by Paul Reinman and Bernard Sachs from Flash 104, February 1949.