Its many years since I've read about the Boeing 707 lower ventral fin but I felt the above explanations were missing a vital ingredient so I got some of my old books out to double check.........
The early B707's without the additional lower ventral fin could achieve attitudes on take-off that were greater than the minimum unstick attitude under certain conditions - so essentially the wing drag was much greater than the early engines could overcome - and the wing was essentially in a stalled attitude - so an early rotation on take-off would prevent further acceleration - and the aircraft would never get airborne. I also seem to remember that early Comets had this same problem and there was a take-off crash at Rome Ciampino (LIRA / CIA) caused by this phenomena.
Combined with directional take-off engine out problems on the B707 as mentioned above where the ventral fin gave a increase fin area and more control it also geometrically prevented the early B707 from getting into the minimum unstick attitude - I think all modern jet aircraft are geometrically restricted in this manner although I understand that modern wing design and more powerful engines make the problem unlikely. I've seen many minimum unstick test films such as the B747 with its tail on the runway still getting airborne. So no lower ventral fin but the shape of the lower rear fuselage providing the geometric restriction.
I'm not an aerodynamics or aircraft design expert so apologies if my explanation is confused or less than perfect. Further reading on this matter is Handling the Big Jets by D.P. Davies 3rd Edition " Mis-handled Rotations" Page 181-184. Various Boeing 707 histories also discuss this problem. Later Boeing 707's had different wings and more powerful engines as well as taller vertical stabilisers and improved rudder boost.
I hope this answer provides some further interest and will no doubt attract a few corrections and further arguments.
Cheers