Along with the unpressurized Model 202, Martin offered a companion pressurized model, known as the Model 303. United Air Lines was the primary sponsor of the 303, having invested one million dollars of its own money in the development. In addition to the 50 slated for delivery to United, Martin also had orders from Northwest Airlines, Braniff Airways, and Pan American-Grace Airways (PANAGRA) for 59 more.
Apart from cabin pressurization, the 303 differed from the 202 in that the wing span was shortened by about 4ft (1.2m) in an attempt to gain cruise performance at the expense of takeoff and climb performance. The 303 made its first flight on July 3, 1947; however, United canceled its large order a short time later.
Because of prevailing market conditions and serious stability problems encountered with the 202 and a need to redesign the wing structure, Martin dropped the 303 program the following December with a reported loss of more than $15 million. The prototype and almost all records were then scrapped for tax purposes.
A total of 148 of the 2-0-2, 3-0-3, and 4-0-4 types were built until 1952.