Steven Hilton announced Tuesday he will step down as chief executive of the Conrad N. Hilton Foundation at the end of 2015. The Agoura Hills charity has an endowment of $2.4 billion. In the last 30 years, it has spent more than $1 billion on philanthropic projects, mostly to provide clean water in underdeveloped countries and combat disease and homelessness. Hilton, grandson of the hotel magnate who created the foundation, has worked at the organization for 30 years, starting as a program assistant. In a letter posted on the foundation’s website, he said it was time for another executive to take the foundation to the next step in its evolution. “I hope to remain involved in this work by continuing my role as chairman of the foundation board of directors,” the letter stated. “We do not foresee significant changes in direction or funding in the near future.” The letter did not discuss the search for Hilton’s replacement. During his tenure, Hilton managed the construction of the foundation’s campus in Agoura Hills and its expansion to more than 50 staff members.