The BBC did a test on an old decomissioned 747, putting a dummy with the same type and amount of explosives found on the underwear bomber. As you can see in the above video, the flexibility of the outside of the frame allowed it to absorb the blast. If the bomb had gone off, the plane would have still been able to fly and land.

Of course, the damage inside the plane still would have been horrific. But it’s nice to know that the trusty 747 can stand up to such abuse.

Gizmodo

The pressurized cabin was also addressed:

Some readers have questioned the validity of the experiment given that the plane was tested on the ground. The programme-makers gave this response:

At the time of the attempted detonation, Flight 253 was descending rapidly and its altitude has been estimated to be around 10,000ft.

At that height the difference in pressure inside and outside the plane would not have been great enough significantly to affect the explosion.

So the explosion team ruled that the decommissioned plane’s lack of doors was not a factor in the test.
With this quantity of explosive, the peak pressure and impulse are over quickly and decay rapidly over distance. The doors were sufficiently far away that the overpressure would have dissipated before it reached them.