Tanks, key weapon in WWII. The Americans rushed the M4 Sherman tank into production. Drawings were complete by March 1941 (well before Pearl Harbor). Pilot model was tested at Aberdeen proving ground in September 1941 , quick work. First production models were coming off the line by February 1942. again quick work. Two hundred Shermans were sent into action with the British 8th Army for the Battle of El Alamein in October 1942. That's a total of 18 months to go from drawings to action.
Upon introduction, October 1942, the Sherman, with a short barrel 75 mm gun was competitive with German tanks. However, the Germans shortly introduced new bigger tanks (Tiger and Panther) which were better than the Sherman. The Germans had thicker armor and bigger guns.
Back in Washington, the Army Ordinance board wanted to introduce a heavier American tank, but Army Ground Forces (the generals in action in the field) feared a loss of production and held the Ordinance folks at bay. It wasn't until the Battle of the Bulge in 1945, where German tanks clearly outclassed the Sherman, that Ordinance got the go ahead to ship the heavy M26 Pershing tank. A few Pershings saw action before the end of the war and it was generally agreed that they were a match for the German Tiger tanks.
During the war US industry churned out 50,000 Sherman tanks, four times the number of tanks built in Germany. During this massive production run, a number of really heavy duty design changes were made. Engines for the first Shermans were 9 cylinder air cooled radial engines. When this went into short supply later production Shermans received twin GMC 6-71 diesels, or the Chrysler multibank 30 cylinder engine, or a Ford V8 of 500 horsepower, or the Caterpillar radial diesel engine of 450 horsepower. Those of us who have done an engine swap in hot rods, are impressed with a production line that can do an engine swap and still churn out 50,000 units on time.
The first Shermans had a bolted together cast nose on a welded hull. Later models had a one piece steel cast hull, and even later models had an all welded hull. Again, impressive redesigns pushed into production with hardly a hiccup in output. The early model short barrel 75 mm gun was replaced by a much longer barrel higher velocity 76 mm gun in American service, and the British had their even fiercer 17 pounder anti gun (75 mm but much higher velocity) installed in their Shermans.
In short, production was able to swallow five different engines, three different hulls and three different guns with hardly a hiccup. They probably could have switched over to the M26 Pershing with little more effort and no drop in output.