In searching for the roots of Design Thinking, I'm currently attacking the issue on two fronts.
The first is my personal experience with DT, the second is a study of the literature.
Since I kept many of the textbooks and notes from my design courses at Stanford, I went back thru them to see what parts of the design process were being emphasized and when, particularly between 1978 and '81, when I was majoring in Product Design.
In those days, the design process was framed as ETC, for Express, Test, Cycle. Bob McKim's "Experiences in Visual Thinking" was the text for ME 101; Visual Thinking, which was the introductory course to the Product Design major. I took that class in the fall of 1977 and took Art 60, Basic Design, in the spring of 1978, which was also when I changed my major ("flipped") from Civil Engineering to Product Design. (and which I'll post about later.)
Going back thru my library, I found several books which I bought for courses in the PD major;
Experiences in Visual Thinking, by Robert McKim (1972 & 80)
The Universal Traveler, by Don Koberg and Jim Bagnall (1972, 73, 74 & 76)
Conceptual Blockbusting, by Jim Adams (also Stanford faculty) (1974, 76, 79 & 86)
Design Yourself! by Hanks, Belliston and Edwards (1977 & 78)
Are Your Lights On?, by Donald Gause and Gerald Weinberg (1982) (not a text, but very good)
Wake Up Your Creative Genius, by Hanks and Parry (1983)
Wake Up Your Creative Genius is particularly interesting. A lot of the material feels like Jim Adam's Conceptual Blockbusting. Wake Up Your Creative Genius has a much looser graphic style but would be a great guidebook for an entrepreneur. Because it is about how to succeed at developing and profiting from your ideas, Wake Up Your Creative Genius pushes well into Design Thinking's current triangle, which includes business viability.
One interesting thing about all these books is that none of them specifically mentions empathy in the index. They all at least touch on the idea that emotion plays a big part in the design process and that the needs of the customer are very important, but IDEO and the d.school have clearly dragged empathy front and center .
Recalling the Google nGrams of how frequently the terms Design Thinking and design thinking were used in the literature; Before 1970, it was nearly zero. From there it rose gradually until about 1982, (0.000,000,025%) where it really started to take off, rising over 1000%, (to 0.000,000,350%) between then and 2000.
For me, this hints that Design Thinking as a formal method wasn't really being discussed much before the early 80's. Before that, from looking at the texts associated with the nGrams, the use of the term seems to have been mostly in the context of studying designers' thought processes generally, whether they be Automotive, Industrial, Product, Architecture or Software... whatever.
I some cases, business was looking for a way to tame the illogical methods of designers (those wily artistic engineers!) and to find more predictable, repeatable, manageable, ways of designing the increasingly complex systems of software and hardware made possible in part due to the explosion in the use of the computer. In others, designers (Van Doren and Archer) were doing a bit of navel gazing in an early effort to try and explain themselves in an effort to build bridges to the C Suite and the Financiers.
So, my current feeling is that the deep, nameless, roots of Design Thinking now go back at least to the 1940's. Although Tim Brown tried to push that back to Thomas Edison, I think the current definition of DT, with its specific emphasis on business viability and empathy, draws a line of demarcation before the post War era of Industrial Design.
The reason I make that distinction is that from my study of Edison, I don't think he consciously advocated empathy in his process. His observations about himself clearly indicate that he was a lot more comfortable with things, and the money which their utility could generate, than the emotions the users of the electric light, telegraph or phonograph felt when using them. Edison was about functional practicality, even in the way he dated and married.
The earliest application of a process focus on touching the hearts of your customer's (empathy), and business which I have found so far, is still Walt Disney. In the early days he did miss out on the business side of the equation. If not for his brother Roy managing the money, I'm not sure Walt would have survived on his own. It just wasn't his primary focus. Money was a way to get things done for him, not something to collect or hoard. It needed to be put to work. Walt wanted to put as much money as possible back into his projects, constantly ran over budget and originally wanted admission to Disneyland to be free. However, he also knew that his guests would gladly pay for a good experience and had first hand knowledge of utter financial failure, so business viability was a very real goal for him.
In any case, the search continues...