It had originally been developed as an airliner, the A.204, but when Aero could not find buyers for the design, it was militarised and successfully marketed to the Czechoslovak Air Force.

In 1936, the Ministry of National Defense (MNO) issued specifications for a new military aircraft, the type III.

[2] The Aero A.304 was a three- to four-seat twin-engine low-wing aircraft of mixed design with a retractable tail landing gear.

[3] A troops and bulkier objects could be transported in the main cabin of the aircraft which was equipped with full-length windows.

These were fitted with all accessories (exhaust headers, NACA annular shrouds, oil coolers, ducts, fire walls) and formed separate, easily removable assemblies.

[3] The aircraft were to be delivered to the Czechoslovak Air Force in the amount of 15 pieces and would belong to its most modern machines.

Romania and Greece also showed interest in them, but in the fall of 1938 negotiations were interrupted as a result of the Munich Agreement.

In 1941, one aircraft, named "Pelikan", was purchased by the Bulgarian Air Force for coastal patrol duty until 1943.