The answer is Boron fiber!
Boron fibers were first discovered in 1959 and were developed during the 1960s into the first true high-performance fibers. Until that time, glass fiber was the only other high-strength fiber available in continuous lengths, and the low modulus of glass severely restricted its use in high-performance structures.
Because of the presence of a dense tungsten boride core, the diameter of boron fibers is significantly greater than that of carbon fibers. This large diameter means that it is virtually impossible to weave these fibers into fabric in the same way that other reinforcements are.
Boron fiber reinforced composites were used successfully in several important aircraft component programs during the 1970s including the skins of the horizontal stabilizers on the F-14 and the horizontal and vertical stabilizers and rudders on the F-15.
There is little doubt that the composites industry would not be what it is today without the maturation of carbon fiber products used in aerospace and industrial applications. But that does not change the reality that boron fiber got the industry started.
