Polymer‐derived ceramics (PDCs) are innovative materials with a wide range of novel applications (micro fibers, protective coatings, porous materials, MEMS). Among high‐performance, non‐oxide ceramics, hexagonal boron nitride offers interesting potentialities as fibrous reinforcing agent for ceramic composites. Boron nitride can easily be produced from preceramic polymers and previous studies have shown the great potential of poly[(B‐methylamino)borazines], called polyMAB as melt‐spinnable precursors for the preparation of this type of ceramics. The goal of the present study is to provide a comprehensive structural and rheological characterization of polyMAB‐type polymers using a combination of thermal, structural, and chemical experiments, as well as rheological investigations and constitutive modeling, in order to predict and control polymer spinnability from the early stage of material formation. The results from dynamic shear rheology are consistent with the rheological behavior observed in spinning. The experimental measurements of the fiber diameter during steady spinning put in evidence the difference in extensional rheological behavior and spinnability between samples. Numerical simulations of 1D model of fiber spinning emphasis the importance of heat transfer at the exit from the die and relevant differences for the polymers under investigation in the evolution of spin extensional viscosity along the fiber. The present work evidences for the first time the rheological behavior of polyMAB samples and the link to their chemistry, especially in relation to the fiber spinning process.