A prototype was originally constructed in 1942, as an experiment, and the design was standardized in June 1944. In July, the European command requested that all M10 battalions be converted to M36s, and the first vehicles reached the front lines in September. It proved more than capable of countering heavy tanks – it was recorded as disabling a Panther tank at 3,200 yards – and its roots in the M10 meant that it possessed greater survivability than the M18. A modification of the M18 upgunned to 90mm was never approved for series production. All the US tank destroyers were built without turret roofs. This was done to save weight but also allowed a wider field of view to spot enemy armor  and quicker ammunition stowage. The drawbacks included vulnerability to small arms fire (especially from elevated positions), grenades, and splinters from air bursting artillery. Exposure to wind, rain, snow and freezing temperatures also made operations difficult, and many tank destroyer crews placed tarpaulins or other material over the turret to improve both their comfort and their operational effectiveness. Under the 1941 scheme, when divisional antitank battalions were converted to tank destroyers, they were renumbered. Those from infantry divisions received a number in the 600 range, those from armored divisions in the 700s, and those from field artillery in the 800s. Towed and self-propelled units were often noted by (T) or (SP) being appended to the number.