Depot in Minneapolis; the, Martin-Omaha manufacturing plant, and the Hawaiian Air Depot at Hickam Field. The Birmingham Air Depot's primary mission was modifying Liberators from Willow Run. Liberator variants produced at Willow Run The B-24E was the first variant of the B-24 that underwent primary manufacture by Ford at Willow Run. Not only did Ford build 490 complete planes, but it also supplied components of B-24Es as kits that could be trucked for final assembly at the factories of Consolidated in Fort Worth and Douglas in Tulsa, 144 and 167 kits. B-24Es built and fully assembled at Ford were designated B-24E-FO; those assembled at Tulsa and Fort Worth out of parts supplied by Ford were designated B-24E-DT and B-24E-CF respectively. Because of production delays encountered at Willow Run as a result of the inevitable difficulties and snags involved in the adaptation of automobile manufacturing techniques to aircraft, the B-24Es produced at Willow Run were, generally, obsolete by the time that it began to roll off the production lines, and most were relegated to training roles in the United States and hence few ever saw combat. The B-24H was the first variant produced by Ford at Willow Run in large numbers that went into combat. The B-24H differed from earlier B-24s by having a second turret placed in the nose of the aircraft to increase defensive firepower. Because of the many structural changes required to accommodate the nose turret, the first B-24Hs were delivered slightly behind schedule, with the first machines rolling off the production lines at Ford in late June 1943. Production for the B-24H at Willow Run was 1,780. Upon the introduction of the B-24J, all three of the Liberator manufacturing plants converted to the production of this version. The B-24J incorporated a hydraulically driven tail turret and other defensive armament modifications in the nose of the aircraft. The bomber plant produced its first B-24J in April 1944; 1587 were built at Willow Run. During June 1944, the Army determined that the San Diego and Willow Run plants would be capable of meeting all future requirements for Liberator production. The Boeing B-29 Superfortress was taking over the long-range bombing role in the Pacific Theater and no new B-24 units were programmed for deployment in the other combat theaters of Europe, the Mediterranean or in the CBI. The B-24L was the first product of the new, downsized Liberator production pool.