I believe that when an aircraft (I'm thinking 777 specifically), does it's first flight, it is called a 'B1' flight. I.e. Boeing 1. If there are any discrepancies or anything that needs to be looked at, it will be rectified after the flight and taken on a 'B2' flight. 'B3
' or even 'B5' flights may be required. Once they are happy, the aircraft is handed over to test pilots sent from the customer.
The aircraft then does a 'C1' flight, with the customer going through a preset list of manouvres and checks. If there are things the customer is not completely happy with, the aircraft is fixed and does a 'C2' flight. 'C3
' or 'C4' flights etc... may be required.
If everything goes very well, I believe they can get away with a B1 and C1 flight, however on average, I believe that 3 flights are done, so on average, a new 777 may have about 7 or 8 hours on it when the aircraft is officially signed over to the customer.
Things are probably a bit different for 757s and 737s, as they have to be flown to BFI
from Renton, and I am not sure if this counts as their B1 flight. Also some airlines do not send their test pilots to inspect the aircraft, instead choosing to trust Boeing's test flights and accept the aircraft without a C1 flight, so their hours would probably be much lower.