A cargo aircraft is an airplane designed and used for the carriage of goods, rather than passengers. This role demands a number of features that makes a cargo aircraft instantly identifiable; a "fat" looking fuselage, a high-wing to allow the cargo area to sit near the ground, a large number of wheels to allow it to land at unprepared locations, and a high-mounted tail to allow cargo to be driven directly into and off the aircraft. The cargo class should be compared to the freight aircraft, which is a conversion of an existing passenger design, which lacks most of these "custom" features.
Aircraft were put to use carrying cargo in the form of air mail as early as 1911. Although the earliest aircraft were not designed primarily as cargo carriers, by the mid 1920's airplane manufacturers were designing and building dedicated cargo aircraft.
The earliest "true" cargo aircraft is arguably the World War II German design, the Arado Ar 232. The Ar 232 was intended to supplant the earlier Junkers Ju 52 freighter conversions, but only small numbers were built. Most other forces used freighters in the cargo role as well, most notably the Douglas DC-3, which served with practically every allied nation.
In the immediate post-war era a number of new custom-built cargo aircraft were introduced, often including some "experimental" features. For instance, the US's C-82 Packet featured a removable cargo area, while the C-123 Provider introduced the now-common upswept tail. But it was the introduction of the turboprop that allowed the class to mature, and even one of its earliest examples, the C-130 Hercules, is still the yardstick against which newer designs are measured.