The M1917 Revolver refers to two .45 ACP revolvers with the same designation used by the United States during World War I. During World War I, the official sidearm of American forces was the M1911 Government .45 Automatic, however supply shortages forced the U.S. Army to ask two major gun manufacturers, Colt and Smith & Wesson, to each update their existing commercial heavy frame revolvers and build a version that could be fielded as a supplement to the standard M1911.
Colt already had an offering to the military, their M1909 heavy revolver, but it was chambered for the antiquated .45 Long Colt round. They re-chambered the revolver for the newly adopted .45 ACP round (calling the commercial variant "the Colt New Service Revolver"). Smith & Wesson re-chambered their own Second Model .44 Hand Ejector revolver as well. Thus there are two versions of the Model 1917 revolver that are externally very similar, one built by Colt and one built by Smith & Wesson.