CAMBRIDGE, Mass. (AP) — An Obama-era effort enlisting startup companies to come up with solutions to the military's toughest technological challenges is funding experimental drones, new cybersecurity technology and advanced communications systems for soldiers.

But as the Department of Defense's "Defense Innovation Unit Experimental," or DIUx office, approaches the two-year mark this month, it continues to face questions from Republican leaders in Congress and others who view it as a still-unproven, and possibly unnecessary, venture.

U.S. Rep. Mac Thornberry, a Texas Republican who chairs the H...