The origin of string theory can be traced back to 1969:
String theory was first studied in the late 1960s as a theory of the strong nuclear force, before being abandoned in favor of quantum chromodynamics.
So, string-like objects were used back then to explain the workings of the strong force. The strong force is nowadays seen in the light of quantum chromodynamics (I typed chronodynamics first and wondered if one can speak of chronodynamics, the dynamics of time). Massless and colored spin-one gauge fields represent the strong force.
As can be seen in the very summit history above, spin 2 particles were contained in the attempt to describe the strong interaction. Quarks and the chromodynamic field were not known yet. The strong interaction between nucleons and mesons (the last of which were used in an even earlier attempt to describe the strong force) was considered as a bundle of strings existing between the particles that take part in a strong-force-mediated reaction. As such, the model could explain the force increasing with distance.
Now, gravitons carry spin-2. The chromodynamic gluon field has spin-1. This field wasn't known back then. Was the string bundle (quark confining flux tubes) somehow associated with spin-2 particles? I found this (in this question):
These higher spins states also have higher masses. Amazingly, when all the possible states are plotted in a spin-versus-mass-squared graph, the result is a beautiful set of straight lines with even spacing between the 2-spin additions. These lovely and highly unexpected lines are called Regge trajectories, and they are the true origins of string theory.
Spin-2 is mentioned here. Is this how the strong-string-bundle in the picture used is related to spin-2? The spin-1 colored gluon field was not known at the time, so spin-2 could be involved, but usually, spin-2 is related to quantum gravity. Tensor particles are related to spacetime distortions. How do they emerge in the early string description of the strong force?