A baby-leaf, red romaine lettuce cultivar designated 40-0203103-B, is disclosed. The invention relates to the seeds of lettuce cultivar 40-0203103-B, to the plants of lettuce cultivar 40-0203103-B and to methods for producing a lettuce plant by crossing the cultivar 40-0203103-B with itself or another lettuce line. The invention further relates to methods for producing a lettuce plant containing in its genetic material one or more transgenes and to the transgenic plants produced by that method and to methods for producing other lettuce lines derived from the cultivar 40-0203103-B.