Convair CV-240 family
The Convair CV-240 is an American airliner that Convair manufactured from 1947 to 1954, initially as a possible replacement for the ubiquitous Douglas DC-3. Featuring a more modern design with cabin pressurization, the 240 series made some inroads as a commercial airliner, and had a long development cycle that produced various civil and military variants. Though reduced in numbers by attrition, various forms of the "Convairliners" continue to fly in the 21st century. Design and development. The design began with a requirement by American Airlines for an airliner to replace its Douglas DC-3s. Convair's original design , the unpressurised Model 110, was a twin-engine, low-wing monoplane of all-metal construction, with 30 seats. It was powered by Pratt & Whitney R-2800 Double Wasp radial engines. It had a tricycle landing gear, and a ventral airstair for passenger boarding. The prototype Model 110, registration NX90653, first flew on July 8, 1946. By this time, American Airlines had changed the requirements to include pressurization and deemed the design too small. Convair used the first prototype for 240 series development work before it had the plane broken up in 1947. To meet the requirements of airlines for a pressurized  airliner, Convair produced a revised design—the Model 240. This had a longer but thinner fuselage than the Model 110, accommodating 40 passengers in the first pressurized, twin-engined airliner. The 240 first flew on March 16, 1947. The Model 240 was followed by the Model 340, which had a longer fuselage, longer-span wings, and more powerful engines. The 340 first flew on October 5, 1951. In 1954, in an attempt to compete with turboprop-powered airliners such as the Vickers Viscount, Convair produced the Model 440 Metropolitan, with more streamlined cowlings, new engine exhausts, and better cabin soundproofing . As the "Super 240" evolved into the CV-340 and CV-440, the design reached the limit of piston-engine performance, and future development centered on conversion to turboprop power. Operational history. Convair delivered the first production Convairliner to American on February 29, 1948. They delivered a total of 75 to American—and another 50 to Western Airlines, Continental Airlines, Pan American Airways, Lufthansa, KLM, Swissair, Sabena, and Trans Australia Airlines. A CV-240 was the first private aircraft used in a United States presidential campaign. In 1960, John F. Kennedy used a CV-240 named "Caroline" (after his daughter) during his campaign. This  aircraft is now preserved in the National Air and Space Museum. After aborted negotiations with TWA and Eastern for "Super 240" orders, Convair temporarily halted 240 series production. In response to a United inquiry, Convair redesigned the Super 240, calling it the CV-340. United ordered 55, and more US orders came from Braniff, Continental, Delta, Northeast, and National. Other orders came from abroad, and the CV-340 was popular in South America. The CV-340 earned a reputation for reliability and profitability, and was developed into the CV-440 Metropolitan, the final piston-engined variant of the Convairliners. Kelowna Flightcraft Air Charter, the  major remaining operator of this model, currently holds the type certificate for this aircraft. Used price for a Convair 240 in 1960 was around £40,000. Variants. "Data from:" General Dynamics Aircraft and their predecessors Civil variants. A stretched Convair CV-5800 of IFL Group with this aircraft being developed by Kelowna Flightcraft (now KF Aerospace) in Canada