B-25 hits the Empire State Building
9/11 wasn't the first time that a large plane had hit a tall building in New York, as this article recalls:
However, these two situations aren't entirely comparable.
The maximum weight of a B-25 ranged from 27,100 lb to a limit of 41,800 lb, for instance (Source). But that's the maximum. This plane wasn't laden with bombs, or heavy on fuel, so would be considerably less (most estimates seem to say it weighed 20,000 pounds).
Meanwhile FEMA said the 9/11 planes had “an estimated gross weight of 274,000 pounds” (Source).
The maximum speed of a B-25 ranged from 275 mph to 315 mph, depending on which version it was, and as the B-25 pilot was flying through fog it's unlikely he would reach that. Cruising speed was 230 mph, and we've seen estimates that the plane was travelling at more like 200 to 225 mph.
By comparison, on 9/11, "American Airlines Flight 11 crashes at a speed of roughly 470 mph" and "United Airlines Flight 175 crashes at a speed of about 590 mph" (http://channel.nationalgeographic.com/channel/inside911/timeline.html).
A quick kinetic energy calculation shows how much of a difference this all makes.
If we say the B-25 weighed 25,000 pounds (11,363.64 kg) and was travelling at 250 mph (111.76 metres per second), then the kinetic energy (KE = 1/2 (M * (V * V))) involved in the collision was almost 71 million Joules. But the energy of impact for Flight 11 was over 2.5 billion Joules, more than 35 times greater; and for Flight 175 it reached more than 3.9 billion Joules, more than 50 times greater than the B-25 impact.
And the differences don't stop there.
The B-25 had a "normal total fuel load of 974 US gallons"; a proportion of this would have been used already in the plane’s flight. By comparison, "it has been estimated that both UA Flight 175 and AA Flight 11 were carrying about 10,000 gallons of fuel when they impacted".
The end result of this was considerably less impact damage, as this photo shows.
Less structural damage meant no real issues in terms of supporting the load of the building above (which was constructed entirely differently from the WTC anyway). The considerably reduced fuel load meant fire was less of an issue, and the blaze that did arise was brought under control without much difficulty:
At first glance it might look like the B-25 crash has some relevance to 9/11, then, but the facts say otherwise. The two events bear very little comparison, and it should be no surprise that they also had such very different outcomes.