The U.S. is giving American exporters a sizable tax break on the goods and services they sell overseas. But the benefit might not be enough to convince Corporate America to expand its U.S. operations beyond what it was already planning.

Instead, the deduction may be going to companies that were already going to export with or without the tax incentive, such as airlines that fly internationally, including United Continental Holdings Inc., and defense contractors like Boeing Co. and Lockheed Martin Corp.

The provision was included in the Republican tax overhaul as a way to prod companies to move along the...