"The Boeing Model 450 B-47 Stratojet was a long-range, six-engined, jet-powered medium bomber built to fly at high subsonic speeds and at high altitudes. It was primarily designed to drop nuclear bombs on the Soviet Union. With its engines carried in pods under the swept wing, the B-47 was a major innovation in post-World War II combat jet design, and helped lead to modern jet airliners.
The B-47 entered service with the United States Air Force's Strategic Air Command (SAC) in 1951. It never saw combat as a bomber, but was a mainstay of SAC's bomber strength during the 1950s and early 1960s, and remained in use as a bomber until 1965. It was also adapted to a number of other missions, including photo reconnaissance, electronic intelligence and weather reconnaissance, remaining in service as a reconnaissance platform until 1969 and as a testbed until 1977." - Courtesy of Wikpedia
Okay, well with a history that spans from 1951 to 1969, it would be hard to pin down when Marx actually came out with this airplane but it was probably mid-1950s as Marx would have wanted to capitalize on the latest and greatest in aeronautical engineering.