As the initial contract was for 300 vehicles, General Motors' Fisher Tank Arsenal produced the last 300 M10A1 tank destroyers in January 1944 without turrets for immediate conversion to M36s. This conversion lasted from April to July 1944. The contract was later increased to 500 vehicles, as it was decided that existing M10A1s were also to be converted to M36s. The requirement was later increased to 600 vehicles on May 15, 1944. As it was found that the M10 tank destroyer had struggled against German tanks like the Panther and especially the Tiger during the Normandy campaign, the contract was increased to 1,400 vehicles on July 29, 1944. This caused problems, as only 913 of the 1,413 M10A1s that had been completed could be requisitioned from training units. Due to the lack of M10A1 hulls, it was decided to finish up the initial production run by mounting M36 turrets onto M4A3 Sherman hulls, with the necessary internal changes. The production of 187 90 mm Gun Motor Carriage M36B1 ran from October to December 1944. From June to December 1944, Massey-Harris converted 500 M10A1s into M36s. From October to December 1944, American Locomotive Company converted 413 M10A1s into M36s. The Army reduced the 1,400-vehicle objective for 1944 to 1,342 vehicles. 350 more conversions were scheduled for 1945; this number was increased to 584. A final batch of 200 M10A1s was converted by the Montreal Locomotive Works in May 1945.