You actually have some pretty good hypothesis here!
To start with, it truly is "a simple engineering matter to attach the wings to the body at an angle such that the right angle of attack for cruise flight corresponded to the body being horizontal", but you missed the design goals slightly. This angle is called the angle of incidence, or the rigging angle of the wing and is the difference between the fuselage angle and the wing angle. This is a very specific angle that is calculated as part of the design of the aircraft.
The wing incidence must satisfy the following design requirements:
- The wing must be able to generate the desired lift coefficient during cruising flight.
- The wing must produce minimum drag during cruising flight.
- The wing setting angle must be such that the wing angle of attack could be safely varied (in fact increased) during take-off operation.
- The wing setting angle must be such that the fuselage generates minimum drag during cruising flight (i.e. the fuselage angle of
attack must be zero in cruise).
These design requirements naturally
match with the wing airfoil angle of attack corresponding to the
airfoil ideal lift coefficient (see figure 5.26).
The typical number for wing incidence for majority of aircraft is
between 0 to 4 degrees. As a general guidance, the wing setting angle
in supersonic fighters, is between 0 to 1 degrees; in GA aircraft,
between 2 to 4 degrees; and in jet transport aircraft is between 3 to
Notice that the proper angle of incidence will have the wing at its most efficient during cruise while the fuselage also generates the minimum amount of drag. Both of these are calculated independently and the angle of incidence is set so that both conditions are satisfied at the same time.
Aircraft design decisions are usually a compromise between different, many times conflicting goals. He goes on to say that it may be adjusted from the optimum in some cases:
The wing setting angle may be modified as the design process
progresses. For instance, a fuselage with large unsweep over the rear
portion to accept aft cargo doors may have their minimum drag at a
small positive angle of attack. In such cases, the wing incidence will
be reduced accordingly. Another, les fundamental, consideration is
that stopping performance during landing operation to get as much
weight on the braked wheels as possible. Thus, there is a benefit to
reduce the wing incidence slightly to the extent that the change is
not felt significantly in the cabin. Reducing the nose gear length
will do the same thing. This technique is limited in passenger
aircraft because a level cabin floor is desirable on the ground. But,
for fighter aircraft, the level floor is not a design consideration.
Another possible reason to adjust the angle of incidence to a non-optimum number is to ensure that when landing the aircraft the nose is not pitched down, in order to avoid a higher likelihood of hitting the nosewheel first on landing.
Most of the material for this answer came from this document written by Mohammad Sadraey at Daniel Webster College