Somewhere in the dark archives of my brain I seem to recall reading that back in the early days of movies a feature was considered any film that ran five reels or longer. A reel in those days was about 10 minutes so we're talking 50 minutes or so. Back then when the studios were cranking out a lot of "B"s many of them only ran between 50 and 60 minutes so I guess that makes sense. I don't know who decided this or how "official" it was.
The obscure Disney animated feature Saludos Amigos is *I think* 47 minutes long. If the Academy defines a "feature" as a film that runs over 40 minutes, that might be why Saludos Amigos is listed as an "animated feature" by Disney, one of the few that was nominated for three or more academy awards.