The “all metal” age arrives. Boeing 221A “Monomail” and P-12 thejivebombers Airliners & Travel January 29, 2018April 20, 2020 1 Minute This view of Boeing’s 221A “Monomail” and P-12 illustrates how quickly aviation technology was advancing in the 1930’s – and things were just getting started. Share this:TwitterFacebookLike this:Like Loading... Related Published by thejivebombers View all posts by thejivebombers Published January 29, 2018April 20, 2020