Was strategic bombing of coastal defenses in France in the days before D-Day effective at reducing German resistance to the invasion?
Great question. Yes and no, mostly no. Hitler's forces had been constructing the Atlantic Wall of defenses along the coast for years prior to D-Day, and it was a complex mesh of pillboxes,underground tunnels, large coastal guns, pre-sighted M-42 machine guns and 88mm weapons, along with tank traps and land mines. The Air Force and the Navy spent considerable time n the week prior trying to "soften up" the defenses. Just as in the Pacific, this mostly had little to no effect at all and most of the structures and guns remained intact on June 6th when the first boats came ashore.
What strategic bombing did accomplish, however, was to limit the ability of Hitler's forces under Field Marshall Erwin Rommel to reinforce the beach, or to move supplies and men along railroads and highways where they might have been able to prevent the landing from succeeding. Most of the success of the invasion has to be given to the Airborne and Army troops of the Americans, British and Canadian forces who stormed the beaches by force.