Convair CV-240 "Super 240"
Originally designed for American Airlines to replace the Douglas DC-3. The first prototype Model 110 registration NX90653 first flew on July 8, 1946. The American Airlines requirement changed to require a pressurized cabin and was deemed too small. The prototype was used for the development of the Convair CV-240, then scrapped in 1947.
To meet the new requirements, Convair built the second prototype Model 240. This had a longer but thinner fuselage capabile of accommodating 40 passengers in a pressurized cabin, the first for a twin-engined airliner. First flight was March 16, 1947. This became known as the Convair CV-240 "Super 240".
The third prototype was the Model 340 with a longer fuselage, bigger wingspan wings and more powerful engines and made a first flight on October 5, 1951. During 1954, to compete with turboprop Vickers Viscount, Convair designed the Model 440 Metropolitan, with more streamlined cowlings, new engine exhausts, and better cabin soundproofing.