The B-1 is a big, heavy bomber that flies fast on a relatively small wing. Getting it off the ground fully-loaded requires swing wings, slats, flaps, and four F101 turbofans with afterburners screaming.
Fun Trivia: Did you know there is a Category E IFR approach category? Approach plates for civil traffic only airports stop at D, so you could be forgiven for thinking that was the last one. Category E is for aircraft with speeds over the threshold between 166 and 210 KIAS. So what are some Category E aircraft? The B-1B Lancer for one. Also the T-38. Previously the SR-71 and Concorde. See, for example, the VOR/DME 29R approach at KFAT, which is a civilian airport but also an Air National Guard base. (Would I want to be circling at 210 knots in less than 3 miles visibility at 1200 foot ceilings? No, I'm good, thanks.)