Combine 3 1/2 cups all-purpose flour and the yeast.
Heat water, sugar, shortening, and salt just till warm (115° to 120°) and shortening is almost melted; stir constantly.
Add to flour mixture.
Beat at low speed of electric mixer for 1/2 minute, scraping bowl.
Beat 3 minutes at high speed.
Stir in whole wheat flour and as much remaining all-purpose flour as you can mix in using a spoon.
Knead in enough remaining flour to make a moderately stiff dough.
Knead till smooth and elastic (10 to 12 minutes).
Place in greased bowl; turn once.
Cover; let rise till double (about 1 hour).
Punch down; turn out onto lightly floured surface.
Divide in half.
Cover; let rest 10 minutes.
Shape each half into loaf; place in 2 greased 8x4x2-inch loaf pans.
Cover; let rise till almost double (about 45 minutes).
Bake at 375° for 40 to 45 minutes.
Cover loosely with foil the last 20 minutes if tops brown too quickly.