I worked on two airplane programs at boeing. The first was a derivative of the 747, the 747-400. The main features of this version that differed from previous 747s were going from analog to digital avionics, a slightly longer body and new (larger) wings. The schedule from project launch to first delivery was about 4 years. We were two weeks late on first delivery.

I also worked on the 787. The difference between the 787 and every previous Boeing plane was almost everything. The schedule from project launch to first delivery was about 4 years. We were two years late.

Moral: don't bite off more than you can chew.