The B-58 program, unlike those for preceding military aircraft, is now recognised as the first weapon system contract. Under this arrangement, Convair acted as the prime contractor responsible for all program elements, not just the aircraft. Convair was required to devise or subcontract everything associated with the aircraft's operation, from the engines to training manuals, spare components, and software, in excess of one million items. Early on, the contract was modified to build a pair of "XB-58" prototypes, 11 "YB-58A" pre-production aircraft, and 31 mission pods including a free-fall bomb pod, a rocket-propelled controllable bomb pod, a reconnaissance pod, and an electronic reconnaissance pod.