Silk fibres are assembled via flow. While changes in the physiological environment of the gland as well as the shear rheology of silk are largely understood, the effect of extensional flow fields on native silk proteins is almost completely unknown. Here we demonstrate that filament stretching on a conventional tensile tester is a suitable technique to assess silk's extensional flow properties and its ability to form fibres under extensional conditions characteristic of natural spinning. We report that native Bombyx mori silk responds differently to extensional flow fields when compared to synthetic linear polymers, as evidenced by a higher Trouton ratio which we attribute to silk's increased interchain interactions. Finally, we show that native silk proteins can only be spun into stable fibres at low extension rates as a result of dehydration, suggesting that extensional fields alone are unable to induce natural fibre formation.