Designed to meet a U.S. Postal Service requirement for a mail plane the MB4 was of unusual design using two fuselages from the MB3 fighter mounted either side of a central nacelle containing the two Wright Hisso engines. The pilot occupied the port fuselage and the co-pilot mechanic the starboard with up to 1,500 lb of cargo stored behind each pilot. The plane flew badly with one fuselage wanting to take off before the other and severe vibration from the two engines. It was so bad that the the general superintendent of Thomas Morse, one Jerome Field described it as "the worst thing on wings". One example was tested by the U.S. Postal Service and found totally wanting and was scrapped in 1921. The U.S.Army may have purchased three but these were not used and stored until suffering the same fate as the first model.