Since you stuck a question mark at the end of your sentence, I assume you're asking a question. The answer is: no, a jetliner would not be clearly visible, because from the view from the Double Tree Hotel, the wall that was hit by 77 was hidden from view. The fireball you see is only what is rising over the roof of the Pentagon. There is also an elevated highway blocking the view. Here's a Google map of the area:
http://maps.google.com/maps?hl=en&ll=38.866127,-77.052999&spn=0.013032,0.027874&t=h&z=16
The Double Tree hotel is clearly marked, south and slightly east of the Pentagon where Army Navy Drive curves south. Flight 77 hit the west-facing wall of the pentagon, which is clearly not visible from the Double Tree.
Here's a video for you:
911 Case Study: Pentagon Flight 77 - YouTube
Towards the end you'll see lots and lots of photos of plane debris right at the scene, including some with obvious markings that match American Airlines markings.
Of all the 9/11 conspiracy theories, this one has always struck me as the most bizarre. I mean, there were dozens of witnesses who claim to have seen a plane hit the Pentagon, but not one credible eyewitness has described a missile hitting it. Wouldn't it make more sense to just fly an actual plane into it instead of going to all the trouble of confiscating every last video tape and scrambling the brains or threatening the lives of everyone within eyesight of the crash? Think about it, man.