Authors who write about American fighter aircraft of the World War 2 era are unanimous in stating categorically there never was a fighter project with the designation "P-74". For some obscure reason, this particular number (and perhaps P-73 as well) seems to have been skipped. The aviation historian James Fahey claims that the P-73 and P-74 designations were deliberately omitted as a result of political pressure applied to the Army by the Fisher Body Division of the General Motors Corporation. In 1942, Fisher was hoping to interest the Army in its new escort fighter design. At that time, the next available Army pursuit designation was P-73, but Fisher wanted the Army to assign to its new escort fighter a "nice symbolic number", something that would sound nice in advertising copy and would make for memorable slogans--something like "The French 75 in World War 1, the Fisher P-75 in World War 2" was envisaged. Fisher got its way and the Army agreed to assign the designation P-75 to its escort fighter project, the designations P-73 and P-74 being deliberately skipped.
In contrast to the P-73, I am aware of no sources that actually claim that there ever was a P-74.