There was an engine of this brand, produced perhaps before the merger of the two companies, and which was considered an auxiliary engine of the Saturn IV-B carrier rocket propulsion system. One of these engines is exposed at the NASM.

"McDonnell-Douglas Rocket Engine"

From Appendix 10: McDonnell-Douglas was also collaborating with NASA on the development of nuclear engines. They presented the following module.

“SNE”

-In collaboration with Los Alamos National Laboratory they designed the reusable SNE or “Small nuclear Engine”.

-The ROOST (Reusable One-stage OrbitalSpace Truck) primary engine was planned for the first stage of the vehicle.

“ROOST primary engine”

“ROOST secondary engine”