I did a simple search and found it in the first listing in Wikipedia
B-24H Because of obvious vulnerability of the B-24 to head-on attack, the B-24H design incorporated an electrically powered Emerson A-15 nose turret. Approximately 50 other airframe changes were made, including a redesigned bombardier compartment. The tail turret was given larger windows for better visibility and the Martin A-3 top turret received an enlarged "high hat" dome. The waist gunner positions were enclosed with Plexiglas windows and laterally offset (as the later B-17G's waist positions had been) to reduce mutual interference between the two waist gunners during battle. Most H model aircraft were built by Ford at the Willow Run factory. (Total: 3,100)
B-24J The B-24J was very similar to the B-24H, but shortages of the Emerson nose turret required use of a modified, hydraulically powered Consolidated A-6 turret in most J model aircraft built at Consolidated's San Diego and Fort Worth factories. The B-24J featured an improved autopilot (type C-1) and a bombsight of the M-1 series. B-24H sub-assemblies made by Ford and constructed by other companies and any model with a C-1 or M-1 retrofit, were all designated B-24J. The J model was the only version to be built by all five factories involved in B-24 production. (Total: 6,678)