Dow Chemical Co. and DuPont Co. announced a merger that would fuse two stalwarts of American industry into a giant worth more than $120 billion and would reshape the chemical and agricultural industries.

The combined company would have about $90 billion in total revenue, based on 2014 numbers, and products ranging from corn seeds to Kevlar fiber to foam chemicals used in sneaker soles. That behemoth would serve as a vehicle for cutting some $3 billion in costs before splitting into three separate businesses 18 to 24 months...