In the case of the B-17 / 24 there were really two major reasons for the split. First, the B-17 was a pre-war production aircraft and required more hours to produce than the 24. The B-24 was an "austire" model that was more adaptable to mass production.
The second reason was, as with the B-29 program, to ensure against a problem with one design or the other. In the B-29 program the competing design was the B-32 Dominator. The B-32 proved to be a failure and the B-29 the success. This dual program approach became a US standard for many aerospace programs from WW 2 on.
With tanks, the system is simpler to produce. Thus, one design was adopted as the standard. Aircraft because of their complexity were best ensured against failure by having 2 or 3 different designs (often having different major components like engines) in production.
An additional reason is that once a company has produced the prototype(s) alot of money is already invested in tooling and special jigs to make the plane. To recoup the cost often a small production run is done as with the B-32.