For the Golden Age hero who was around earlier than you may have realized!
First appearance (as just "Tex Thompson"):
all the way back in Action Comics #1 (June 1938)
Changed the name of the feature to "Mr. America":
Action Comics #33 (February 1941)
NOTE: By issue #43 (December 1941), the feature name was changed to "Mr. America and Fat Man", with "Fat Man" being Tex's sidekick Bob Daley.
Finally, there was the change from "Mr. America" to Americommando in:
Action Comics #52 (September 1942).
The last issue the feature appeared in was Action Comics #74 (July 1944).