The production of weapons in World War Two shows the tension between the need for quantity and the need for innovation. Better weapons and more performance are always wanted, but production lines have to be stopped or slowed to allow modifications. This was true even when modifying existing products, like the Spitfire fighter. Moreover, entirely new weapons like the V2 rocket and the atom bomb needed huge investment and locked up important scientific talent that could have been spread over many other projects.
In general, Britain and the USA were relatively conservative in aircraft production, aiming for huge outputs but subtly incorporating ongoing modifications. For example, the Rolls-Royce Merlin aero engine went from 1000hp in 1939 to 2000hp by 1945 through intensive experimental development and the incorporation of new features into the production lines in a highly controlled and imaginative way. The administration of these changes, the production and supply of spare parts for each new mark of engine, and the supply of new or modified instruction manuals and technical notes throughout the RAF was one of the major unsung organisational achievements of wartime production. The historian Jonathan Zeitlin regards the British wartime aircraft programme as having achieved an almost ideal balance between quantity and quality that anticipated the flexible manufacturing systems of today.
By contrast, Germany found it less easy to integrate scientific research into the aircraft production programme. It did use advanced engineering science to initiate ambitious high-tech projects (such as the V2), but this did not have a decisive influence on the course of the war.