Combine bread flour, spelt flour, sourdough starter, oil, and salt in the bowl of stand mixer. Begin to knead using the dough hook. Add water until all the ingredients combine and begin to pull together; you may not need all the water. Knead on low speed for 5 minutes.

Fold the dough into a ball. Place inside an air fryer-safe baking dish. Cover with plastic wrap and allow to double in volume, 5 hours to overnight.

Preheat the air fryer to 390 degrees F (200 degrees C). Remove the plastic wrap and score the loaf.

Place the baking dish in the air fryer and cook until loaf is browned, about 20 minutes. An instant-read thermometer inserted into the center should read at least 190 degrees F (88 degrees C). Allow to cool before slicing.