The concept of Design Sprints has existed forever. Just because Google gave it a name after their successes with Gmail, doesn’t mean that people didn’t know about this or used this. Maybe this was the first time for Silicon Valley. Here are a few different references from earlier than 2015 that might help your research

https://martinfowler.com/articles/lean-inception/

https://www.thoughtworks.com/insights/blog/3-ways-add-design-thinking-inceptions

https://info.thoughtworks.com/ebook-actionable-innovation

Polymath: dad, husband, co-founder, strategist, Computer Vision enthusiast, visual thinker and dog lover.

Polymath: dad, husband, co-founder, strategist, Computer Vision enthusiast, visual thinker and dog lover.