John Bowman

Even though two planes have already crashed in New York, some people in the Pentagon initially think a bomb has gone off when their building is hit:
Steve Carter, who is in the Building Operations Command Center on the first floor of the Pentagon, hears a “big boom,” and tells his assistant, “I think we just got hit by a bomb.” [Vogel, 2007, pp. 434]
John Bowman, a retired Marine lieutenant colonel, is in his office near the main entrance to the Pentagon’s south parking lot at the time of the attack. He later describes, “Most people knew it was a bomb.” [Pentagram, 9/14/2001]
Army Colonel Jonathan Fruendt is in his second floor office in the Pentagon’s inner A Ring, when he feels and hears “a very sharp jolt and the sound of an explosion.” He later recalls, “I thought it was a bomb that had gone off.” [Marble and Milhiser, 9/2004, pp. 73]
Apparently only a few people in the Pentagon initially guess a plane has hit the place. According to the Defense Department’s book about the Pentagon attack, among the few exceptions are Peter Murphy and his companions in the Marine Corps Office of the General Counsel, located on the fourth floor just above where the building is hit: “Unlike most other survivors, Murphy and his companions ‘were pretty certain it was a plane and it was a terrorist,’ even though they had not seen the plane coming in. They had been watching the attack on the Twin Towers and had speculated about such an attack on the Pentagon.” [Goldberg et al., 2007, pp. 45]