[akiselev]

The Pegasus rocket used to launch satellites from a modified 747 for between 1990 and 2016 [1]. It just doesn't work out well in practice because the cost of the modified plane far exceeds the cost savings from a first stage unless you're doing half a dozen or more launches per year.

[1] https://en.wikipedia.org/wiki/Pegasus_(rocket)

[isostatic]

Virgin Orbital are doing something similar - satellite launching from a 747.