WHAT WAS EUROPE, DADDY? (via Greg Mankiw):

U.S. Share of World GDP Remarkably Constant (Mark Perry, 11/19/09, Carpe Diem)

Somewhat surprisingly, the Economic Research Service of the U.S. Department of Agriculture has some great international historical macroeconomic datasets. According to its website:

The International Macroeconomic Data Set provides data from 1969 through 2020 for real (adjusted for inflation) gross domestic product (GDP), population, real exchange rates, and other variables for the 190 countries and 34 regions that are most important for U.S. agricultural trade.

The chart above shows the annual shares of real world GDP for four geographical regions (European Union 15, Asia/Oceania, Latin America and the combined share of Africa and the Middle East) compared to the U.S. share of world GDP between 1969 and 2009 (data here). What might be surprising is that the U.S. share of world GDP has been relatively constant for the last 40 years, and is actually slightly higher in 2009 (26.7%) that it was in 1975 (26.3%). It's also interesting that the EU15's share of world GDP has declined from about 36% of world output in 1969 to only 27% in 2009. Further, despite having a large share of the world's oil reserves, the Middle East's share of global output has increased from only 2.23% in 1969 to 3.16% in 2009 (graph shows Middle East combined with Africa).