Related Rule
United States of America
Practice Relating to Rule 19. Control during the Execution of Attacks
The US Air Force Pamphlet (1976) states:
An attack must be cancelled or suspended if it becomes apparent that the objective is not a military one, or that it is subject to special protection or that the attack may be expected to cause incidental loss of civilian life, injury to civilians, damage to civilian objects, or a combination thereof which would be excessive in relation to the concrete and direct military advantage anticipated. 
United States, Air Force Pamphlet 110-31, International Law – The Conduct of Armed Conflict and Air Operations, US Department of the Air Force, 1976, § 5-3(c)(1)(b)(ii).
The Report on US Practice notes that “during the 12-day bombardment campaign of 1972, the crews of B-52 heavy bombers took a number of steps to minimize civilian casualties in the heavily-populated Hanoi and Haiphong areas”. 
Report on US Practice, 1997, Chapter 1.6.
A published account of these events states:
The instructions to the RNs [radar navigators] were that if they were not 100 percent sure of their aiming point, “then don’t drop; bring the bombs back” … We had been briefed not to make any evasive maneuvers on the bomb run so that the radar navigator would be positive he was aiming at the right target. If he was not absolutely sure he had the right target, we were to withhold our bombs and then jettison them into the ocean on our way back to Guam. We did not want to hit anything but military targets. Precision bombing was the object of our mission. The crews were briefed this way and they followed their instructions. 
James R. McCarthy and George B. Allison, Linebacker II: A View From the Rock, US Air Force Southeast Asia Monograph Series, Volume VI, Monograph 8, 1979, pp. 46–47, 50 and 59–64.
In 1991, during a news briefing concerning the Gulf War, the US Secretary of Defense stated: “The pilots of the allied air forces have operated in accordance with clear instructions to launch weapons only when they are certain they’ve selected the right targets under correct conditions.” 
United States, News Briefing by the US Secretary of Defense and the Chairman of the Joint Chiefs of Staff, Washington, 23 January 1991, annexed to Letter dated 25 January 1991 to the President of the UN Security Council, UN Doc. S/22168, 29 January 1991, p. 3.
In 1992, in its final report to Congress on the conduct of the Gulf War, the US Department of Defense stated:
Where required, attacking aircraft were accompanied by support mission aircraft to minimize attacking aircraft aircrew distraction from their assigned mission. Aircrews attacking targets in populated areas were directed not to expend their munitions if they lacked positive identification of their targets. When this occurred, aircrews dropped their bombs on alternate targets or returned to base with their weapons. 
United States, Department of Defense, Final Report to Congress on the Conduct of the Persian Gulf War, 10 April 1992, Appendix O, The Role of the Law of War, ILM, Vol. 31, 1992, p. 622.
In 1993, in its report to Congress on the protection of natural and cultural resources during times of war, the US Department of Defense stated: “Aircrews attacking targets in proximity to cultural property were directed not to expend their munitions if they lacked positive identification of their targets.” 
United States, Department of Defense, Report to Congress on International Policies and Procedures Regarding the Protection of Natural and Cultural Resources During Times of War, 19 January 1993, p. 205.