On the bomb-run 

To be conveniently engaged in doing something of grave importance at the moment when another duty calls, so that you have the perfect excuse not to deal with the latter duty. Especially pertinent when dealing with the latter duty may well lead to loss of face/humiliation/failure on your part, but also when the latter duty is merely something that for whatever reason you just do not want to have to deal with.

When one is 'on the bomb-run' they have a totally legitimate excuse.

The phrase relates to the scene in the WWII film Memphis Belle when the pilot’s flask of tomato soup explodes during a flak attack and splatters its contents all over the pilot, the co-pilot and the surrounding cockpit. Mistaking the tomato soup for blood, the duo and top turret gunner are convinced that someone has been hit, so they call the bombardier up to check them out, as they all believe him to be a medical doctor. However, the bombardier has been over exaggerating as he actually only attended two weeks of medical school prior to enlisting, therefore having to deal with any casualties would immediately highlight his incompetence as a medic and loss of face/humiliation would ensue. Thus, the bombardier replies nervously and dismissively ‘Hey, I’m on the bomb-run!’ He is indeed on the bomb-run and therefore has the perfect excuse not to go up-front and play doctor.

1) Guy A: ‘Hey, dude. Can you demonstrate your 1000 consecutive push-ups with perfect form now?’
Dude (trying to finish his assignment for tomorrow, pointing at the pile of papers and text books surrounding him): ‘Hey, I’m on the bomb-run!’

2) Guy A: ‘Hey, dude. There’s that celtic princess. Go tell her how you feel, man!’
Dude (dashing to submit that assignment, deadline in 2 minutes): ‘Hey, I’m on the bomb-run!’

3) Guy A: ‘Hey, dude. The Jehovah’s witnesses are at the door. Can you get it?’
Dude (pulls fully-loaded 6’x6’ bookshelf over on top of himself and lies underneath, desperately trying to prevent the immense weight from crushing him): ‘Hey, I’m on the bomb-run!’

On the bomb-run by Papa J-Bomb November 6, 2012