To meet the growing need of passenger and mail air services, aeronautical companies worldwide, from Boeing to Douglas, developed numerous models that could carry more mail and passengers across great distances, in luxurious comfort of air-travel.

Curtiss Jennies JH-4
These biplanes were heavily used by America during World War I and was a particular favorite for barnstorming. After the War ended, surplus Jennies that have not been destroyed in battle, were operated by Army pilots to deliver mail. However, many of them had taken its toll during the war and their airworthiness was severely compromised.

deHavillands
The initial batch of deHavillands were actually meant as combat-aircraft during World War I, but because the aircraft was only rolled out towards the end, many deHavillands were never used at all. Hence, it became economically feasible to use these deHavillands as mail-planes. However, despite having a good range of 350 miles and generous load, the deHavillands could have been described as a pilot's worst nightmare or "flying coffins".
The sardine-like ergonomics of the plane was such that the pilot was sandwiched between the engine and the mail compartment, which was a safety hazard as it made evacuation during a crash cumbersome.
The pilot often had to bear with the exhaust fumes blowing into him from the exhaust stack in front of the cockpit.

The compass headings often were misleading.
The altimeter could not function below 1000 feet.
However, an improved version of the deHavilland DH-4 was introduced. The flaws listed above were corrected, and minor refittings were done. In the end, the deHavillands proved to be efficient workers and delivered more than 775 million letters in one year alone.

Stearman
The Stearmans were the answers to the growing demand for aircraft that will outdo the aging fleet of Curtis Jennies and the deHavillands. They boasted of better horsepower, and specially-fitted compartments that could carry large amounts of mail in 1 trip, approximately up to 1000 pounds. "Model 4" of the Stearman aircraft family was known as the "Junior Speedmail" as it carried mail and cargo faster and more efficiently than any other plane, due to its Pratt & Whitney high-performance engines.

Airmail Unlimited
History | Types of Mail-Planes | The Brave Pilots | Airlines and Airmail

Airmail Technology Today | What the Airlines Say