The Douglas M-Series of mail planes were developed simultaneously with the O-2 observation planes for the U.S. Army. In a historical way, if not in an engineering sense, these planes were forerunners of the DC transports Douglas would build later, and the companies that flew the early airmail routes would later grow into the airlines that pioneered regular passenger service. On April 27, 1926, a Western Air Express M-2 mail plane flew from Los Angeles to Salt Lake City in the first regularly scheduled airmail flight. A month later the first two paying passengers made the flight. The Douglas M-2 open cockpit biplane cruised at 110 mph and flew no higher than 12,000 feet. However, this wood and fabric craft was a marvel for its day. Powered by a 420 horsepower Liberty V-12 engine, the M-2 could carry 1,000 pounds of mail and two passengers. The passengers rode in the cargo compartments, which were forward of the pilot. They sat on boards, attired as their pilot in helmets, goggles and the warmest clothing they could find. They paid $90 for the opportunity to ride with mail sacks around them and on their laps from Los Angeles to Salt Lake City. It took six hours plus to make that ride in 1926. Running in a series from M-1 through M-4, Douglas produced a total of 59 mail planes between 1925 and 1926.