US stamp design started to get serious in the 1950s. On March 21, 1957, the Post Office Department established the Citizens’ Stamp Advisory Committee. The Federal Registry described the committee’s purpose thusly:
The Stamp Advisory Committee shall advise the Post Office Department on any matters pertaining to the subject matter, design, production and issuance of postage stamps.
This group of seven men (of course) were responsible for the subject, look, and character of US postage stamps.