As many as 400 veteran New York City officers assigned to desk jobs in the Police Department will return to the streets this summer as part of a broad push by Commissioner William J. Bratton to head off a seasonal spike in crime.

The program, known in the department as “Summer All Out,” will last 90 days and draw on uniformed officers and detectives in office assignments such as counterterrorism and personnel work. It was announced in a letter from Mr. Bratton to his top deputies dated Monday and obtained by The New York Times from a person who is skeptical of the strategy outlined in the document.

While the department has periodically reassigned officers to patrol posts to address short-term needs, the length and scope of the redeployment appeared to dwarf those seen in recent decades. It comes amid heightened concern over a rise in shootings in the first six months of the year, up roughly 8 percent over the same period in 2013, and follows months of political jostling over the appropriate size of the 35,000-member department.

“They’ve done ‘all outs’ in the past, a day or two a week, or a day or two a month,” said Roy T. Richter, the president of the Captains Endowment Association, the union representing the upper echelon of city officers. “But nothing to this extent in my memory, and that goes back at least 20 years.”