"U.S. exports and imports,1869-2005 The figure shows U.S. exports (black) and U.S. imports (red), each expressed as a percentage of total output. Exports and imports need not be equal in each year: U.S. exports exceeded imports (shaded gray) during much of the twentieth century. During the 1980s, 1990s and early 2000s, however, U.S. exports were smaller than U.S. imports (shaded red). Sources: Imports and exports of goods and services: 1869-1959 from Historical Statistics of the United States, Colonial Times to 1970, pp. 864-865; 1960-2005 from FRED database, Federal Reserve Bank of St. Louis, research.stlouisfed.org/fred2/series/BOPX and BOPM; output is from Fig. 1.1."
The output data from Fig. 1.1 comes from an article by Christina Romer.
“The Prewar Business Cycle Reconsidered: New Estimates of Gross National Product, 1869-1908.” Journal of Political Economy 97 (February 1989): 1-37.
The percentages seem a little higher than other sources, though. Click on the graph to see a much larger version.