The Ha 139 floatplane was designed as shipboard mailplane. It was a large, clean monoplane with an inverted gull wing and four diesel engines. The Ha 139 could be launched by catapult from the ship when this was near to its destination, thus delivering the post well before the ship was in port. The three built were later pressed into military service, and the third prototype was extensively modified, with a long glazed nose, for maritime reconnaissance.