The Mid-Atlantic Accent

The Mid-Atlantic accent dominated motion pictures and radio from the 1930s through the 1950s. It was encouraged because it was thought to be a non-regional, nondescript accent. Since it seemed to be located somewhere between Britain and America, it became known as the Trans-Atlantic accent. So what's the story behind this accent? How did it start? Why did it become so popular? And why did it mostly disappear by the 1960s?