The Convair 240 / 340 / 440 family of airliners was produced between 1947 and the mid 1950s as a successful attempt at a DC-3 replacement. It was the first twin engined, pressurised airliner. After a false start, the configuration of two R-2800 engines and 40 seats was settled on – hence the “240” as the model number. An interesting feature of the 240 series was the “air-stair” at the tail, similar to that which became normal on the DC-9 and Boeing 727.  First flight was in March 1947. The success of this design can be seen by the numbers of updated airframes still flying commercially around the World.
The 1950s saw the design refined and enlarged into the CV-340 to provide greater range and accommodation with 44 seats. The CV-440 was a further refinement intended to compete with newly introduced turbo-props however in its piston engined form, the CV-440 “Metroliner” had reached its limit.  When production ceased in the mid ’50s, nearly 1200 aircraft in this family had been produced.
The US military bought a number of the Convair 240 / 340 family – with the C-131D and later being 340 variants purchased between 1954 and 1956. A “Mid-Life” update program was applied to many to bring them to “440” standard and they are covered in the appropriate FAA manual as Convair 440 aircraft.