World War II-era military planes were some of the most advanced and efficient combat machines. Whether a pure fighter and interceptor or a heavy bomber, thousands of planes of each type were produced. The Consolidated B-24 Liberator is one such aircraft, the most produced heavy bomber. Designed and developed by US-based Consolidated Aircraft, the aircraft performed its first flight in December 1939 and entered United States military service in 1941. More than 18,000 examples of the B-24 Liberator were produced between 1940 and 1945. At least two out of a handful of surviving B-24s are still airworthy.