After World War II, Boeing primarily was a military airplane company. The company began to expand back into commercial airplanes and pursue new fields of missiles and spacecraft. The engineering department divided model numbers into blocks of 100 for each of the new product areas: 300s and 400s continued to represent aircraft; 500s would be used on turbine engines; 600s for rockets and missiles; and 700s for jet transport aircraft. For the first airplane, the marketing department decided that "Model 700" wasn't catchy enough, so they picked Model 707 to start.