[2] The O-340 was designed by Lycoming specifically for the TEMCO-Riley D-16A Twin Navion project.
Jack Riley, the designer of that aircraft was interested in an upgraded version of the Lycoming O-320 that would produce more power to give the Twin Navion a better single-engine service ceiling.
The engine was later used in a number of airplanes and helicopters and also in amateur-built aircraft designs.
All have a displacement of 340.4 cubic inches (5.58 litres) and the cylinders have air-cooled heads.
[2] The O-340 series was certified under Type Certificate E-277 and first approved on 20 July 1954.