The Pentagon has prepared a specific plan for a preemptive strike on North Korea's missile sites should President Trump order such an attack.
Two senior military officials — and two senior retired officers — told NBC News that key to the plan would be a B-1B heavy bomber attack originating from Andersen Air Force Base in Guam.
Pairs of B-1s have conducted 11 practice runs of a similar mission since the end of May, the last taking place on Monday. The training has accelerated since May, according to officials. In an actual mission, the non-nuclear bombers would be supported by satellites and drones and surrounded by fighter jets as well as aerial refueling and electronic warfare planes.
"Of all the military options … [President Trump] could consider, this would be one of the two or three that would at least have the possibility of not escalating the situation," said retired Adm. James Stavridis, NATO's former Supreme Allied Commander Europe and now dean of the Fletcher School at Tufts University and an NBC News analyst.
Six B-1B "Lancer" bombers are currently positioned in Guam, 2,100 miles by air to North Korea. Military sources point out that the battle tested B-1, a workhorse for the past 16 years in both Afghanistan and Iraq, has been modernized and updated — "doubled in capability," according to the Air Force.
The target set, multiple sources say, would be approximately two dozen North Korean missile-launch sites, testing grounds and support facilities. The sources told NBC News they feel confident they have accurately identified a set of relevant targets. They say that the months-long standoff between North Korea and the Trump administration, together with North Korean activity and testing of a wide variety of missiles since January, has helped them to refine their understanding of North Korea's web of missile facilities.
On Wednesday, the Pentagon released a written statement from Secretary of Defense James Mattis reiterating American military readiness for both offense and defense.
"While our State Department is making every effort to resolve this global threat through diplomatic means," the statement said, "it must be noted that the combined allied militaries now possess the most precise, rehearsed and robust defensive and offensive capabilities on Earth."

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"Diplomacy remains the lead," said Gen. Terrence J. O'Shaughnessy, the U.S. Pacific Air Forces commander, after the B-1 bombers' late May training run. "However, we have a responsibility to our allies and our nation to showcase our unwavering commitment while planning for the worst-case scenario. If called upon, we are ready to respond with rapid, lethal, and overwhelming force at a time and place of our choosing."

Asked about the B-1 bomber plan, two U.S. officials told NBC News that the bombers were among the options under consideration but not the only option. These officials insist that action would come from air, land and sea — and cyber.
"There is no good option," a senior intelligence official involved in North Korean planning told NBC News, but a unilateral American bomber strike not supported by any assets in the South constitutes "the best of a lot of bad options."