Many in the automobile industry believed that aircraft engineering design could be frozen which would allow greater production rates. They learned that this was not possible. The aircraft industry design and manufacturing were undergoing great upheavals. From the end of World War I with canvas covered wings made from wood aircraft were now aluminum framed and covered. With this trend came streamlined fuselages and other new features demanding newer design and manufacturing techniques.
Additionally, changes were required by the military services as they learned what worked or didn’t work, what need updating to meet an enemies’ new capability, or to take advantage of an enemies’ weakness. Therefore, engineering hours such as those on the North American B-25 did not peak prior to the first unit’s production as was common in high production run industries.
Design Change
A complicating factor for World War II weapons platforms was production often began prior to completion of design. In fact, continual design change was a norm for the industry. Unfinished aircraft were piling up in airframer factories as they could not incorporate all the engineering changes during the assembly process. Therefore, the assembly lines as well as engineering developed with ways to accommodate this dilemma.
Solutions
Two solutions arose the first was to incorporate changes group together in blocks to streamline the assembly process as much as possible. The second was to complete the aircraft assembly process elsewhere in modification centers. Aircraft manufacturers were not used to modifying aircraft after they had left the assembly lines, but their airline customers were. The airlines had repair and maintenance facilities with the appropriate mechanics, hangars, tools, and airfields to accomplish these updates.
Modification centers offered a compromise solution maintaining the assembly lines production flows while being able to incorporate changes needed by the Air Force customers. Some modification centers were able to tow the vehicles from the factory lines to their hangars. The Ford B-24 Willow Run facility in Michigan modification center was in Birmingham, Alabama requiring the aircraft to be at least flight worthy.
Although a convenient solution, modification centers were an untimely and costly solution. Repairs and modifications were done with hand tools versus production tools or jigs and much grinding and shimming occurred, driving up labor costs. Modification center use did not diminish after the initial war production start up but continued heavily throughout. Several planes had almost as many modification center labor hours as factory assembly line production hours.
This change dynamic required engineering support in design, manufacturing, modification centers, and in the field. An issue identified early in the requirement or design phase might take a few hours to resolve. But that same issue identified during manufacturing or out in the field could take many multiple of those few engineering hours to resolve and in addition to that would be the factory or modification center workers additional hours.
North American B-25 Engineering Hours 1940-44 Data
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Source: Holley , Irving Brinton, Jr. “United States Army In World War II Special Studies Buying Aircraft: Materiel Procurement For The Army Air Forces.” 1964, Page 528, Center of Military History United States Army, Library of Congress Catalog Card Number: 64-60000, https://history.army.mil/html/books/011/11-2/index.html, Data accessed on July 12, 2022
Other Production Data
US Military Civilian Aircraft Market 1926-36
Production Runs of Soviet Aircraft, 1941-1945
Warplanes Produced by Nation 1940 to 1945
US War Cost of Production by Category
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