Douglas DC-6 (1946-1958): Nearly identical to the DC-4 in appearance, the DC-6 was first conceived as a long-range military transport. The plane made its maiden flight on February 15, 1946. The key difference between the DC-4 and DC-6 was the DC-6’s larger engines, which were twice as powerful as the engines on the DC-4. The DC-6 was the first Douglas airliner with a pressurized cabin, which enabled it to fly higher and faster, thus taking its passengers “over the weather.” The DC-6 was the most economical commercial aircraft of the propeller era. Its operating costs were among the lowest until jetliners came on the scene. Military versions were produced under the designations C-118 (Air Force) and R6D (Navy). President Harry Truman chose a DC-6 for official use, and christened it the “Independence,” for his hometown in Missouri. During its 12-year production run, Douglas delivered 537 DC-6s, plus 167 military versions.