General readers enjoy counterfactual histories, “what if” scenarios that rewrite history. Academic historians seldom write explicit counterfactuals, despite their value in isolating the causes and contingencies that shaped events. Surprisingly, historians of technology have ignored this analytic tool, even though firms and engineers commonly considered alternative designs and actions while developing a product or technology. This article provides a “constrained counterfactual,” comparing two designs for bridges across the Mississippi River at St. Louis, both proposed in 1867: the Eads Bridge, and the Boomer/Post bridge. It covers three topics: exploring the conventional narratives on the Eads Bridge (completed in 1874); comparing the Eads design to that of the Boomer/Post alternative; and offering a counterfactual service life for that proposed crossing. The article seeks to isolate why James Eads’s design and his company succeeded, and to show the analytic value of counterfactuals for historians of technology.