The U.S. Air Force plans to start testing a new heavy bomber in 2037. There are several interesting things to note about this plan. First, there's the obvious fact that the air force is planning that far ahead. Second, 2037 will be the 100th anniversary of the acceptance of the B-17 "Flying Fortress," the first modern heavy bomber, by the U.S. Army Air Corps. The third item of note is that the air force is going so long without having a heavy bomber in development. Over a sixty year period, from the early 1930s to the early 1990s, there were fifteen heavy bombers developed. Only 13 of them actually entered service (B-17 in 1939, B-24 in 1942, B-29 in 1944, B-32 in 1945, B-50 in 1947, B-45 in 1948, B-36 in 1948, B-49 "Flying Wing" cancelled in 1952, B-47 in 1952, B-52 in 1955, B-58 in 1960, FB-111 in 1969, B-70 development halted in 1966, B-1 in 1985 and B-2 in 1992.) 

Finally, the design of the new bomber is unclear. Anything goes.  The current fleet of B-1, B-2 and B-52 aircraft is aging faster than expected because of heavy combat use. The success of JDAM smart bombs dropped from heavy bomber circling overhead for hours, means that the existing heavy bombers will probably be used more, and wear out faster, than expected. So the current defense budget provides $45 million to begin design work on a new heavy bomber. This is liable to be a contentious issue, with the growing popularity of UAVs and suggestions that a modified commercial aircraft might be more cost-effective as a heavy bomber.