The Potez 840 was an all-metal low-wing cantilever monoplane with a retractable tricycle landing gear. It had a crew of three and a cabin for 18 passengers. Powered by four 440shp (328 kW) Turboméca Astazou II turboprops two mounted on the leading edge of each wing. The prototype first flew on 29 April 1961, a second aircraft flew in June 1962 and had more powerful 600shp (447 kW) Turboméca Astazou XII engines. The second prototype carried out a sales tour of North America and it was planned to build a batch of 25 aircraft for Chicago based Turbo Flight Inc. but only two more prototype aircraft were built, one for static testing. The next two aircraft were designated the Potez 841 and they were powered by 550shp (417 kW) Pratt & Whitney Canada PT6A-6 turboprops. Two more modified Aztazou-powered aircraft were produced, one in 1965 and one in 1967.
It was intended to build Potez 840s in a factory in Baldonnel in Ireland, built with financial aid from the Irish Government, but this factory was closed in 1968 without completing a single aircraft.
The intact fuselage of a second can be found on the island of Shetland. It suffered a wheels-up landing at Sumburgh Airport in 1981. Many years later the fuselage was recovered and moved to its current location.